No, a 1978 305 Chevy van transmission will almost certainly *not* work in a 1993 Chevy 4.3L pickup. Even though both are automatics, the bolt patterns, input shaft diameters, and overall dimensions are highly unlikely to be compatible. The 1978 transmission is a much older design, and the 4.3L in the 1993 truck uses a completely different transmission. The computer interfaces alone would be a massive problem.
You'll need a transmission specifically designed for the 1993 4.3L engine. Attempting to force a mismatch could cause significant damage to your truck.
