Paul M, I'm doing just such a frame swap on my 64 SS conv right now. I bought a used Elky frame, sandblasted it,measured the old frame vveerryy carefully,cut the replacement frame and rewelded the rear section back on, all the while making sure everything was level and straight. I also had the benefit of my brother-in-law's 65 frame laying nearby to use as a guide whenever I had any doubts. If you know someone that welds for a living, get them to help you, because they are usually good at fabrication. As you take things apart, mark them as to where they came from so you'll know how to put them back. Get an assembly manual if you dont already have one. I do auto and truck repair for a living and I didn't think it was that hard,It is a time consuming job and I wouldn't recommend it to the faint of heart or those with limited skills. I personally think a convertible is worth the time and effort to do this kind of repair..Kind of long, but I wanted you to get an idea of what you are getting into before you got started. I hope I didn't discourage you from doing it. hope this helps.
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Leo Paugh
Maryland Chevelle Club #017