It isn't going to get any easier either...
Are you wanting to use the 76 and 81 parts together. If so, it won't work (without some MAJOR WORK) Each unit will have to be installed per year (that is the best explanation). IE: if you want the heater from the '81, install it complete component wise ONLY from the '81
This is my suggestion as if you were starting from the beginning. Strip the '81 cab down to nothing but the cab shell (minus windows unless you are replacing them) Then, go to the '76 and pull everything out of it and put it in the '81. This will make the '81 basically a 76 Cab. This is the approach I would do if my '76 cab was bad and I needed a new one.
The electrical connection/wiring and the A/C are the main groups for the cab, and they are the ones that will cause you the most pain. If you don't have to, I wouldn't swap these main components. (keep in mind I have no idea what your trying to accomplish or the salvagability of either truck.)
You might have to enlighten me some. I hope I am helping more than hurting....