Chipy - If saving your car is that important to you it can be done. You said you want to start by fixing a frame that sits too high. I was in the same situation w/a 1941 Ford conv't that I bought w/out a frame. I had to weld 3"x4" tubing from the front portion where the frame bends out(1972 Chevelle frame) to the rear portion where the frame end its bend back in. I then cut the outriggers off thus, ending up w/a frame w/straight rails. To lower the frame I cut the pocket out of the lower control arms & lowered them 3/4" which lowered the car 1 1/2" in the front. At the rear I cut the upper frame spring mount out & raised it 1 1/2" thus,lowering the back 1 1/2". There was a article in Streetrodder mag in 1989 or 1990 on how to do this. It was writen by the owner of the Rod shop that now is in Indiana.
As far as the floor being cut .....if the doors open & close correctly you are okay, if not you will have the mmove the lower portion of tthe floor in or out to get proper door alignment & bolted to the frame before welding the floor back in. With a sedan body I do not think the car panels moved.
You will end up w/a very nice driving car. If you are intent to proceed I will try to find a few pictures of the finished frame & E-mail them to you good luck on what ever path you chose to follow.........Don