What year is my truck!?

[Tdubbyount]

I've always believed its a 79 Cheyenne...that's what the registration says. However the VIN on the registration says its a 76 GMC. That doesn't make sense because it was originally a 5.7 diesel, which came out in 78 I believe...? It has all the 79 Cheyenne trim, interior was correct for a 79, but the partial vin that I could find on the cab matched the 76 vin on the registration. Really confused on this one...
Anyone know of other hidden vin numbers I can find? I want to know which parts of my truck are from where.
After the restoration it has no resemblance of the truck that it was. I put a pre-78 style bezels and trim. Engine, tranny and all running gear have been swapped for earlier 3/4 ton stuff. I just would like to know for sure what year my cab, frame and possibly bed are.

Here's a picture of before I started.