One difference in the cab is the seatbelt anchor points
I think the 76/77 cab has the separate shoulder belt anchor higher up the door post than the lap belt anchor
And the newer cabs had belts with both anchors in the same retractor
Someone chime in and correct me if I’m wrong
Regarding VIN plates , Swapping cabs is a don’t ask / don’t tell kind of thing
If you just remove the VIN plate from the new replacement cab and install the VIN plate from the old rusty cab , which in most states is technically against the law , but don’t tell anyone
Then No one will ever know ,
If your 77 has it in the door post I would just keep it and move it to the replacement cab and make sure the replacement cab doesn’t have any VIN on the windshield to conflict or confuse any cops
So it’s like the if a tree falls in the forest but no one is there to hear it , did it make any noise when it fell ?
If , however , you move or alter a VIN plate for the purpose of fraud , I’m 100% sure that it’s against the law