The rear is a 14 bolt, and the front is a 3/4 ton 10 bolt.
You might want to check into potential registration problems putting the older cab on. It looks like it is a 2wd C10, which could cause problems when you get it inspected, and its a 4x4. If I was in your shoes, I'd be leaving the 4x4's cab right where it is, and swapping the older front clip, wiring etc over.
The newer cab has a different interior, with the VIN plate in a slightly different location on the dash, so you'll probally want to run the 81's interior. The 81 and the older 76 should have nearly identical wiring, except the 76 will have an amp gauge, while the newer truck will have a volt gauge. Just swap the 76's instrument cluster into the 81, it has the nicer 100 mph speedometer anyways
Bolting the older front clip on is easy too, just take the cowl off the newer cab, and take the wiper linkages out, then swap the older truck's linkages on, and use the newer core support's body bushings on the older core support. Everything else bolts straight up
