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Look up forum member BarryK and find the link to his website. He has dimensional drawings of all of the C3 frames on the site. If there are differences in the later model year frames I'm betting they are minor and easily adaptable to your year.
You can do it, but there will be some changes. One of the biggest is the gas tank probably won't fit. The 75 my son bought as a project car has a frame from a 79 and the gas tank was a pain to get fitted. Had to add a second cross support. Definitely bubba'd, but he just wants to autocross it so he doesn't care too much about the looks. Also noticed that the rear bumper cover doesn't fit properly. There may be more, but that's the biggest problems we ran into.
Look up forum member : DWncchs. His name is Roger, i know he had a 76 frame for sale a while back, he may still have it.
I don't know what area you,re located at? He's in Dayton Ohio.
frame was in good shape.
Good luck!
Tony.