* Bolt Pattern: The transmission's bellhousing bolt pattern, which connects it to the engine, is almost certainly different between a 1984 and 1991 vehicle. Even if they use the same basic transmission type (e.g., TH350, 700R4), the changes over seven years could mean incompatibility.
* Computer Control: 1991 transmissions were likely electronically controlled, while the 1984 model might have been mechanically controlled or had a simpler electronic system. This would require major modifications to the wiring harness and possibly the engine computer.
* Output Shaft: The length and spline count of the output shaft (connecting to the driveshaft) are likely different and incompatible.
* Torque Converter: The torque converter will also be different and incompatible.
While some transmission swaps are possible with adapters and modifications, the differences between a 1984 and 1991 Chevrolet transmission are likely too substantial to make this a practical or cost-effective project. It would be far easier and cheaper to find a used automatic transmission that's a direct replacement for the original 1984 El Camino transmission.
