i wouldn't worry about rust, i'd worry more about pinhole leaks from the solder. coating a tank is always a good idea, at least for longevity purposes. and as long as you're riding the bike, the tank won't rust. with fuel in it, it'll be fine. it's when they sit and the condensation gets into it that you'll have a problem.
well, here's some updates on my bike. some progress, some going backwards...
my skiptooth chain is dying. i've broken it twice, just from pedaling it to start. it was NOS, but i think the 60 or so years of inactivity are taking it's toll. haven't done any real damage, just had to remove and replace broken links.
the spring collar that holds the rear sprocket on the hub snapped, causing the sprocket to jam up with the chain, and scratched up some spokes. nothing bad, just cosmetic. replaced the collar with one from the many hubs i have laying around, and fixed it.
now for the big one.
the chain tensioner...
once my sprocket wore in, the engine drive side was too loose. there's no way to tighten both chains up, and believe me, i tried. i made another rear cog for the pedal side out of a 20 tooth, added a link and a half to the chain, but then it was too loose. went back and forth with sprockets and crap but nothing worked. either one side or the other was too loose or too tight.
if it wasn't a skiptooth chain, i would just put another odd toothed cog on the rear, like a 17 or a 19. but with the skiptooth, it has to be an even number of teeth. follow me so far?
so, i spent all day making this: