The only differences would be the gauges and the sending units. If you have idiot lights over true gauges the sending units will be different for sure. There could be some minor wiring differences from year to year but that usually would be if you went backwards in years instead of forward. So If you have a 81 and find a 82 or 83 just make sure the plugs are the same. My 81 had different plugs throughout the vehicle over later years. 82's and up had similar plugs to 2nd gens. in the engine compartment but I never had to pull my Speedometer. Now if you plan on keeping the current cluster and just want to swap out the actual speedometer I would say that if it looks the same on the outside the mount points would be the same.
This is exactly what i want to do but if i can get an actual cluster it would be better
