* Different body styles: While seemingly minor, the difference between Fleetside and Stepside could affect the transmission's fitment due to chassis differences. The frame, driveshaft length, and even the location of transmission mount points might vary enough to cause problems.
* Different engines: The transmission's compatibility is heavily dependent on the engine it's paired with. If your 1989 Fleetside and 1990 Stepside have different engines (e.g., V6 vs. V8, different displacement), the transmissions will almost certainly be incompatible. Even if the engines are the same, the transmission might have different gear ratios or bolt patterns.
* Internal components: Even if the external dimensions seem compatible, the internal components of the transmissions could be different, leading to gear ratio mismatches or other mechanical problems.
Before you attempt anything: You absolutely need to confirm the transmission model number from both trucks. This number (usually found on a tag on the transmission itself) will tell you definitively if they're compatible. Then, you can consult a parts catalog or mechanic to verify. Swapping the wrong transmission could cause significant damage to your truck.
