After re-cutting the threads it was obvious that the sender was held in by 4M .70 metric nuts. Once I replaced the missing nuts I was able to pressure test the tank and find that there were no leaks. That was a very exciting discovery. Most of my simple projects on this car usually require some sort of extra effort. Like patching the radiator (which was done), scraping rubber off the floor in the passenger compartment or finding missing pieces.
It took me less time to wire brush and prep the tank for rust reform than it did to find the proper nuts for the sender unit. Who knew! I used Rustoleum Rust Reformer which has gotten pretty good reviews online. We shall see. Ideally I would have media blasted this tank and then painting, but it was too large for my cabinet. I think this application of product should protect the tank for years to come. The previous owner had already sealed the inside of the tank. If it begins to leak eventually I will use a two part gas tank epoxy to fix it.
For now, enjoy the pictures.