The frame is one of the first things I addressed. In fact the undercarriage of my 62 is way nicer than the body and interior.

I'm kind of obsessed with it since it's the nicest area on my truck. As soon as an area pops up under there I address it. The frame, axles, have been completely painted and if an area pops up I clean and repaint it. I use Satin Black Rustoleum for this purpose. One thing I recommend is coating the frame internally
with this. The stuff works great. I think I ordered about 4 to 5 cans. I used the extra to do the inside of the doors and rear quarter panels and even in the rocker panels.
Since I can't afford to do a proper paint job at this time I have gone around and through the body and POR 15'd any rust areas. I have three holes in the body, one on each rear quarter panel and one on the end of the inside drivers door. I prepped those and POR 15'd them two years ago and they are still rust free (technically). My goal was to stop any further rusting until I could get a proper paint job which will of course cost thousands. So far it's worked as the body is essentially rust free, even though as mentioned it has three 1/2 inch holes, the rust is not active. It's sealed. Maybe this approach will work for you until you can afford to do it right. So far it's worked for me.