Can a 1984 Chevy three-quarter ton automatic 4WD transmission be used in 1978 truck?

It's highly unlikely a 1984 Chevy three-quarter ton automatic 4WD transmission will be a direct bolt-in replacement for a 1978 Chevy truck. While both are Chevy trucks, the transmission designs, bolt patterns, output shaft lengths, and even the transfer case interfaces changed significantly between those years.

You'd face significant challenges, including:

* Different bolt patterns: The transmission case itself might not bolt to the 1978's engine or frame.

* Different input shafts: The input shaft connecting to the torque converter might not match the 1978 engine's flexplate.

* Transfer case incompatibility: The transfer case from the 1984 truck is almost certainly incompatible with the 1978's setup.

* Wiring harness and controls: The electronic controls for the 1984 transmission are likely different and require extensive rewiring.

* Length differences: The overall length of the transmission might not fit within the 1978 truck's frame.

In short, while some parts *might* be adaptable, it's a major undertaking requiring significant fabrication, custom parts, and potentially specialized knowledge. It would likely be far more cost-effective and simpler to find a transmission from a 1978 or very close-year Chevy three-quarter ton 4WD truck.