Thanks for all the suggestions. There was one local old guy who did rad and gas tank boil-out, but unfortunately he decided to retire earlier this year. I'm on the search for another such service. If anyone in West Michigan has a tip about a likely shop, I'd appreciate it.
In the meantime, I've removed the tank and started the removal process of rust. There was absolutely no liquid in the tank (given that it was last on the road in 1990, that's not too surprising). Haven't done the nuts, bolts and agitation process yet. Emptied out about a cup of rust by moving the tank around after blowing then vacuuming with shop-vac. Then a simple rinse out with some kerosene, drained it out through the small drain plug on the tank bottom, then dried it out with shop-vac blowing for about ten minutes.
Are the fuel level senders still available? Mine has a decent enough float, but it looks like the windings on the rheostat are broken. No idea if it will register anything when I get the tank reinstalled.
Also, on the bottom of the tank there's a small drain plug. Removed that one with no issue. But there's a larger brass opening, which I assume is where you get access to the in-tank filter. The fitting has a square female indent that looks to be just about 11mm or so (yes, I know the 140's didn't use metric for much). Anyone know what tool I'd use to fit into this opening and then gently work it open with my ratchet?