This is a 79 Chrysler Cordoba, no frame to speak of. We strengthened it by welding a pair of u-type signposts as half-*** subframe connectors. Works great!
Its just a $100 car that we bought specifically for this purpose. Firstly, I would get a 2 door, 4 door is gonna be tougher. Also, it pretty much has to have pillarless doors.
Mostly, don't bother pretending you are gonna make it nice. Especially if its a 4 door, cutting the roof off is easy, making it nice is not. That and the GM "framed" cars of that era had pathetic excuses for frames, basically they were unitbodies with bolt-in subframes, that happened to be connected with some really thin c channel. Its gonna flop around. Either way, I'd use this as a learning experience.
We started by just using a recip saw with a nice long blade to take out the back pillars first, and then used a tape line above the windshield as a guide for the front cut. doing it again, I would remove the windshield, ours cracked. It has a small crack to start off, but when we were done, it went across the whole thing.
Strength is OK, not railroad trestle, but no stability probs, doors open fine, etc.... This car was actually "balanced" in that when resting the roof had no stress on it, so when resting its all straight, when its on jackstands, the doors stick a bit.
Anyway, good luck!