Compatibility Issues:
* Bolt Pattern: The bellhousing bolt pattern on the transmission must match the engine's. Using a transmission with a different bolt pattern will require extensive modification or fabrication, which is usually not practical.
* Torque Converter: The torque converter is specific to the transmission and engine combination. Using the incorrect converter will likely cause damage.
* Computer/ECU: The transmission control module (TCM) interacts with the engine control module (ECM). Swapping to a significantly different transmission might require reprogramming or significant wiring modifications, which is challenging.
* Driveshaft: The length and yoke configuration of the driveshaft need to be compatible with the new transmission.
In short: Sticking with a used, original 4-speed automatic transmission from a similar-year Mercury Cougar V6 is the most straightforward and reliable approach. Anything else will require considerable mechanical expertise and likely custom fabrication. Checking the transmission code on the side of your existing transmission will help you find a precise match when sourcing a replacement.
