Your results may vary, but I used POR-15 gas tank sealer on a previous truck, only to have it fail a few years later and clog everything. Since then, I've decided to just replace the tank when it's an option. I doubt a 64 tank is available, but you never know. Just gunk? No problem. If the gunk pulls away and reveals pin holes, try to get them soldered, but avoid the putty solutions.
Pulled the tank and pressure washed it after the acetone. A ton of grit and funk came out of the tank so trying to do it in the truck would get about half of it. Also cleaned under the tank and painted it with the most horrid of colors (spa blue). Good thing no one will see that!
New float (the 1966 FJ45LV has the same pickup with a shorter arm. The 1965 FJ40 is a different stlye so the LV must have carried over the older parts.