Instead of looking for a direct replacement, you should focus on these:
* A used 4T65E-HD: This is your best bet for a reliable and straightforward swap. Look for one from a similar year Grand Prix GT with a comparable engine. Ensure you source it from a reputable junkyard or used parts dealer, ideally with some warranty or guarantee.
* A remanufactured 4T65E-HD: This offers a better chance of reliability than a used unit but at a higher cost. Several companies specialize in remanufacturing transmissions.
* Repairing your existing transmission: If the damage is not too severe, rebuilding or repairing your existing 4T65E-HD might be a cost-effective solution.
Attempting to use a different transmission type is strongly discouraged unless you have extensive transmission expertise. The bellhousing, torque converter, computer interface, and other critical components are likely incompatible with other GM transmissions, even those from other GM vehicles. You'd need extensive modification to the vehicle itself, potentially including the engine mounts, drivetrain, and even the vehicle's computer system. This is a very complex undertaking and not a feasible DIY project for most.
In short, stick to the 4T65E-HD or its remanufactured/rebuilt equivalent. Anything else would require an extensive and potentially impossible adaptation.
