Here's why:
* Different Bellhousings: The bellhousing, the part of the transmission that bolts to the engine, is different between the S15's engine and the Caprice's engine. Even if both are V6s, GM used different bellhousing patterns across its various engine and transmission combinations.
* Transmission Input Shaft: The input shaft of the transmission (the part that connects to the engine's crankshaft) will likely have a different diameter and/or spline count.
* Possible Differences in Clutch: Even if you could magically make the bellhousing fit, the clutch system (pressure plate, disc, throwout bearing) from the S15 would not be compatible with the Caprice's engine and flywheel.
In short, you'd need a considerable amount of custom fabrication or the use of an adapter plate (which are rare and difficult to find for this specific combination) to even attempt to make this work. It's far easier and more cost-effective to find a transmission that's designed to mate with the 4.3L V6 from the Caprice.
