Sorry if this sounds silly or obvious, but I thought I'd put it in here in case anyone is about to lose their favorite car.
No matter how good any of us are at fixing stuff, if the frame goes, you can usually kiss the thing goodbye, or at least only prolong its demise. Check this site out - www.onlinemetals.com
They have lots of metal you can order there in whatever form, including stainless steel. It's not the cheapest stuff in the world, but not, by any means, a regretable purchase if you are trying to save a cool car.
I noticed 90s like my own tend to have a tendency to rot in the wheel wells, for example. You can get a little stainless in plate form, stick weld it into (or have someone do this part for you...I might buy a welder too when I become loaded ) whatever part of the frame has a hole, or so much rust that you can punch it (obviously clean the hell out of that first). I like to first overlap it a little and adhere it with silicone so it fills in gaps and hinders moisture building up underneath....then weld. Then bondo, sand, prime, and paint. It'll probably be even stronger than it originally was, and the stainless will be a more fruitful solution, as it won't continue to degrade no matter how much you try. Granted, you still can't stop it all, but even some of the thinner metal used as a sheet, you can make some nice splash guards out of to retard the spots that get the worst abuse.