Thanks to you all for taking the time to share your thoughts. Actually, since my original post, I had revised my thinking from using Flex Seal to using something like rubber cement. I think contact cement, being a close cousin to rubber cement, would be an even better option - great adhesion plus remaining flexible for a long time. Internal air pressure will be my friend, however I see centrifugal force being an enemy. I had even thought about applying some to the shoulder of the valve stem to better my chances of an airtight seal there. I couldn't believe Dave suggested that also!

So, besides the four wheels that are still on the car with rotting tires mounted, I have three spare rimes, which will be sand blasted, bead sanded smooth, and painted silver, just like the four on the car. One of those spare rims will get a new tire mounted and balanced just like the four that will be on the road. The other two spare rims will sit in my shed looking pretty until I hit a good ole' Atlanta pot hole or clobber a curb and need to replace one on the car.

I belong to a small automobile hobby club where have a sand blaster, a tire mounting machine, and a computer balancer, among other equipment. I have already stripped the old (maybe 25-30 years old) tires from all three spare rims and started the clean up process. I put each tireless rim on the balancer and ran it by itself to verify trueness and balance before wasting my time mounting a tire on any of them - all look good except for my concern about finding inner tubes within them and wondering about possible spot weld leaks.

Per Derek's comments I compared the valve stem hole diameter on all three spares in addition to the ones still on the car. They all seem to measure just a smidge under 1/2" diameter so I believe the 0.453" dia x 1-1/2" long valve stems that I purchased from Amazon will be a good fit. It was encouraging to find the wheels on the car have tubeless valve stems, leading me to believe the rims don't leak. I think I still want to coat the spot welds with contact cement just to make sure there's no leakage.