Installing a five speed transmission into an early Mustang, Falcon, Cougar, or almost any early Ford could not be any easier these days. Many of the five speed transmissions found in late model Mustangs can be installed with little modifications. There are several good five speed transmissions to choose from. The Borg Warner/Tremic T-5, Tremic 3550, 3550II TKO, TKOII. T-45 transmission found in 1996 and later mustangs could not be used in early Fords as the 4.6L bell housing has a different bolt pattern. All of the transmissions offer a 5th gear over drive to reduce engine rpm and for improved mileage while weighting less then their automatic or four speed counterparts.
There are three ways to go installing a five speed transmissions into early Fords. Using an early four speed bell housing, late T-5 or Tremic bell housing, or after market scatter shield. Which one you decide depends on what you are starting with and how you are going to use the car.
For a 65-66 Mustang I would recommend a T-5 transmission for engines making under 450hp. T-5 fits much better than a larger Tremic 3550 or TKO due to clearance issues with the transmission tunnel. Tremics transmission can be installed but it requires modifying the transmission tunnel.
Automatics Transmissions Equipped Cars
Start by using a T-5 bell housing assembly from an 1984-93 mustang 5.0L and a Modern DriveLine clutch cable kit. This is the easiest way to solve many problems. The T-5 bell housing fits both the Windsor block and the T-5 without use of an adapter plate. A 65-66 clutch cable conversion kit removes the need to use the 4 speed clutch linkage and provide greater header clearance with smoother operation. The cable kit bolts to the stock clutch pedal in just minutes.
Using the T-5 bell housing with a clutch cable kit will require using a diaphragm pressure plate found in late model Mustangs. Engines dated 1981 and earlier will need a 157 tooth 28oz imbalance flywheel. Later engines will need a 157 tooth 50oz imbalance flywheel or otherwise know as a 5.0L flywheel.
T-5 have ¾” longer input shaft which sets the transmission back ¾”. Since it is set back ¾” and has a lower transmission mount, a replacement T-5 transmission cross member is needed. Not all cross members are built the same. Most cross members on the market don’t provide enough exhaust clearance or proper mounting of the e-brake lever. The cross member Modern Driveline cross member solves these problems. There is enough clearance to fit convertibles where exhaust systems clearance is very tight. It also is built to withstand just about any type of use. The cross member is designed to use the C-4 transmission mount so don’t make the mistake and purchase a late model T-5 transmission mount as it will not work.
The drive shaft will have to be shortened about ¾” to make up for the ¾” back set. This can be done at any driveline service. While the drive shaft is being shortened, have it serviced with new U-joints. Afterwards the drive shaft is checked for balance. The shifter on a T-5 lines up with the shifter hole in the floor. Just a little of the floor has to be removed on the front and right side of the opening. The T-5 shifter has two threaded holes to mount almost any kind of shift lever desired. Most vendors offer a stock looking shifter lever that bolts up to the shifter. For those who like the Hurst look of the 60’s, there are several chrome levers to choose from. Hurst also offers five speed pattern shift balls in white and black.
Be sure to save the automatic transmission speedometer cable, as it will be reused. You will however need to install a T-5 speedometer driven gear, which are available in a variety of tooth counts to match your differential gear and tire combinations. Installing the new speedometer driven gear is simple, it requires removing the c-clip and pulling the old gear off and sliding the new one on.
Automatics have a neutral safety switch, so that the car can only be started in park. For a manual you do not need this, and the switch must be by-passed or the motor will not start. On early Mustangs the four-plug harness sticks out of the firewall above the master cylinder. Two of the wires control the reverse lights; the other two control the neutral safety switch. Splice two of these wires together and you've bypassed the neutral switch.
The starters from early 289-302 Fords with automatic transmissions work just fine. The starter from any '85-'99 Mustang 5.0, auto or manual, will work. For when header clearance is an issue 1992-95 5.0L Mustangs use a high-torque which will run about $120 more.
Manual Transmissions Equipped Cars
When the car is equipped with a 3 or 4 speed manual transmission, the job is a lot easier than converting from an automatic. There are two ways to install a T-5 transmission for a stick equipped car.
One way is to use the original bell housing and install an adapter plate. This will allow the clutch and clutch linkage to be used. An adapter plate is needed for two reasons. The bolt pattern is different then the four-speed transmission and makes up the depth needed for the longer T-5 input shaft. Make sure to purchase correct adapter plate depending whether you have a five bolt bell housing or a six bolt bell housing. 260 and early 289 are five bolt while later 289-351 are six bolt.
The second way is to use a T-5 bell housing mentioned in the auto-equipped cars. This will eliminate need for an adapter plate. However, the throw out lever needs a folcrum adapter kit to offset the ¾” deeper T-5 bell housing. Using the T-5 bell housing does leave the option installing a cable conversion kit later.
I highly recommend a clutch cable kit is it provides greater header clearance and operates much smoother. It also solves the problem of the Z-bar bending and popping out of it pivots point under heavy loads. When using a cable conversion a T-5 clutch lever, throw out bearing, block plate and clutch lever cover will be needed in addition to the T-5 bell housing. The cable kit comes with a firewall support bracket to minimize firewall flex and provides a positive stop for the cable housing. The support plate is equipped with a tool free cable adjustment. The support plate can be using with manual or power booster.
Manual equipped cars have much longer speedometer cables. The manual cable will have to be replaced with an automatic cable. The manual speedometer cable is routed to go to the passenger side and enter the four speed from the right side. Install the automatic cable through the hole in the lower firewall near the e-brake cable and plug the other hole with a body plug.
1967-70 Mustangs all the same options as the 1965-66 Mustangs. There is however there are three additional five-speed transmissions available from Tremic, 3550, 3550II TKO, TKOII. These transmissions are larger and heavier than a T-5 or four speed and require more transmission tunnel room found in these larger cars. The Tremic 3550/3550II has a longer standard 10 spline input shaft. The TKO/TKOII have a 26 tooth input shafts. These transmission will require a 26 tooth disk, which are available. All Tremec models have the same bolt pattern as a four speed or toploader. If the Tremic input shaft is shortened to match a toploader it will bolt to the early four speed bell housing without the use of an adapter plate. Not needing an adapter plate also means there is no backset on the transmission. The drive shaft length remains unchanged. Shifter location on a Tremic lines up with the opening in the floor. The Tremec transmissions rear mount are further back then a four speed, so 1967-70 mustang Tremec replacement cross member is needed.
In 1995, Ford used a Tremic 3550 behind a 351 Mustang Cobra R Model. This same bell housing or scatter shield can be used for a early mustang small blocks, using a Modern DriveLine clutch cable kit. The 1967-70 clutch kit bolts in the same way the 1965-66 kit does using the stock clutch pedal. To use a cable system on a big block, a four speed scatter shield can be modified to work. More on that later.