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If I read your question correctly, if I you want to install a rear bumper from a 78 for an 80 you'll have to swap out the rear bumper rail too. Not sure about the front.
To fit an 80's front you will need to do some cutting of the fenders to get it to fit. Not sure what else but probably mods to allow fixing as well.
I like the look of the 80's fronts and thought there was a company that made a bolt on type of thing but when I looked in to it, I was told I would need to cut the front up. If you find a bolt on kit, let me know!!!!
The front fenders on the 80s flare out to meet the larger bumper. You'll need to cut the front fender halves off and bond in some for an 80s vette. Not worth the work IMO. But J&D sells the pieces to do it.
I went with an 82 rear bumper and a 74 front on my 78. I just filled in the small gap on the 74 bumper to line up with the body.