Get this, I have a '35 Chevy sedan...but it's 800 miles from where I'm presently living, so I can't even look at it to try and give any advice.
I think your body is supported with wood framing, and in most cases that is largely gone. It seems that one decent way of fixing the problem is to fabricate new jambs out of metal. If you are a competent wood worker, you can replace the wood without a lot of fighting.
Try Googling for your door jamb fix...I know there is an article out there somewhere on exactly how to fabricate the whole thing using metal from scratch. Just can't remember where.