Gawrsh that nice rounded river pebble looks comfy and cool!
A couple of things -
POR-15 likes some rust to bite into.. I used a dilute oven/grill cleaner to get the caustic action going to make it rust and help strip a majority of oil off it but I also let it sit out and weather for a few weeks. Sand-blasting removes too much good metal in my opinion, BE SPARING where you use it - the POR stand for paint over rust, just remove the flakes and scaling that are or about to become loose. If you havent used POR-15 before you are in for a treat
The best way I found to remove the factory asphalt based frame paint remaining everywhere not rusted away was using a 3-inch wire wheel on a compressed air die grinder, beware thrown wires; I wore one for 4-5 days under my skin from lying on one wrongly, taped a magnet over it to get it out. The abrasive biscuits work also but are much slower and load up fast. Did I say beware loose wires? I made up rare-earth magnet broom and religously swept the area everytime I crawled out from under trailer after getting pierced.
The asphalt based paint traces remaining will not solvent wipe away easily - for the important parts (axle plates, F/R exposed frame) I used aerosol carb cleaner but the brown-black paint kept oozing out of the pores of the metal. I ended up over coating some smears, etc and it did just fine.
The ladder frame interior cross member spaces forward of the axle were pristine on my '73 so I used Rustoleum flat-black aerosol there.
I used thinned POR-15 using their solvent (<5%) everywhere else for 2 coats minimum, more coats (4, 5, etc.) where I saw it needed it as I went along. Using a 2" wide brush the extra solvent made it go on easier, cut through any films left after above procedures, and sure stretched the coverage plus helped paint fill the million dimples and pits of serious rust without just forming a bubble that would later pop and leave bare metal.
Getting the second coat on while the first still has some 'grab' to it when you slide a fingertip across it is important, I worked well after midnight using a fluorescent drop light as the only light source and got a paint job 1000% better than I expected. After 2+ years there are only 3 or 4 places where one rust pit did not get filled and has a 1mm rust spot showing.
The first coat fumes are not bad - its when it is firing up to set AND you have to do the second/third coats that it can cause the next day hangover when you've breathed it for 3 or four hours. Stay upwind as much as possible or use a cartridge style respirator.
Well - sorry for the lecture, time to get back to work here... Good luck with the next steps!!