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I suspect the front and rear of the frames differ because of the impact supports and the bumper mountings. Not sure about the legality. Are you from a smog inspection state? Do you intend to title it as a 77 or an 81?
The vin tag on the dash dictates the year as far as most states are cornered. The frame does not matter unless you have a salvage title car that needs to be inspected to be registered.
Check with the DMV in your state, tell them what you have in mind, and ask about the legalities.
And if you don't get the correct answer from the first DMV employee that you talk to.....go back the next day and talk to a different one---it depends on which person that you talk to as to what is legal/what is not!!!!
DMV and smog would be interested in the VIN on the body FAR more than the vintage of the frame.
Think - what do you do a Carfax on; the body VIN or frame VIN?