Removing Fuel Tank - Super H (never done this before)

An elderly neighbor asked me if I could remove the rust from the fuel tank on his Super H. I do a fair amount of rust removal on old tanks around my neck of the woods, so this isn't an unusual request. Though I can do an OK job getting the rust out of the tank with it mounted, I prefer to work on tanks removed from their equipment if at all possible and practical. The owner isn't quite up to much physical effort anymore so he isn't able to remove the tank himself. So I said I'd take the whole tractor and see what I could do. So his son delivered the tractor to my barn.

So here's my situation:

The tank uses a three-point mount, two in front and one in the rear. I thought it would be a (relatively) easy process to remove the nuts on the three mounting studs and remove the tank. Well, it's not quite that easy as the rear stud simply turns when I attempt to back off the nut. The other two studs seem to be OK.

Is there a trick or technique to hold the stud to remove the nut?

I'm sure I can bungle my way though this, but I thought I'd ask more knowledgeable & experienced folks here before I get too aggressive or creative. Maybe there an bit of a trick to this, I don't know but I thought I'd ask.

Thanks for your time...

The mounts do in fact have springs and rubber bushings of some kind... I can clearly see them.

I wasn't sure how the studs were mounted to the tank, that's why I asked about this. I surmise from the responses that the stud is threaded into a T-nut or something similar.

Gordo