I'd recommend sandblasting as well. If you can't get someone to do the whole thing for you, you might pick up one of these:
Sandblasting gunI'm assuming you have an air compressor (at least a 5 HP unit) to run the gun.
That gun can be used to get at the crevices and spots that you can't reach with a rotary tool.
The floor pan has a bunch of pits in it and many of them are all the way through, even if you can't tell from looking. If you try to patch it, you'll find yourself chasing the holes till you get to solid metal.
If you're going to use epoxy primer, do NOT use ospho. The residue from ospho is not compatible with epoxy.
Carl's recommendation of Rustoleum rusty metal primer and top coat is one avenue. I restored a Model A using that back in the dark ages.
If you have pieces that you can dunk in a pan, or keep it wet with paper towels, Evaporust Liquid works well. They also have a gel product, but I have not had much luck with that.