81 and 84 245 compatability

Virtually all the parts are compatible, very little changed until '86.

Assuming the OP is in U.S.A, the '81 would have a 2.1 litre engine with mechanical fuel injection, the '84 would have a 2.3 litre version of the same engine with electronic fuel injection. Factor in the different transmissions and resulting rear axle ratios, and virtually everything in the drivetrain is different. Also, the bumpers and bumper trim would be completely different, but that would be obvious. The rest of the body parts would be interchangeable, as would everything to do with the suspension.

They're all interchangeable though, you can swap bumpers and trim between the two, axles, driveshaft, transmission, engines, you can even swap the cylinder heads between the engines if you wish to change the car from one fuel system to the other. Yes there's differences, but what I was implying is that the body and interior is identical between the two cars.

Right, but the more common use of a parts car (at least in my experience) is swapping small, easy to change parts, like injectors, control units, sensors and trim parts. Not too many people have the time, skill and determination to do the kind of swaps you've done.