'72 Jimmy replacement tank

[bhemingson]

I finally got my replacement gas tank last week. It had the stubby filler neck so I decided to swap necks with my original. We used an air chisel and cut off the the section with the old neck. The old tank had two 3/8 nuts on the studs. They came off easy. We tried to melt the solder with a propane torch and it wasn't working so we moved to oxy/acetylene and the solder melted in under a minute and the neck came off.

My new tank is a Counter Part tank from Taiwan. The nuts on the stud were sort of 8mm and wouldn't budge. We heated them until the studs glowed red and no movement. We wire wheeled off some of the paint and it looks like they brazed the neck on and the nuts to boot. I heated it until I was afraid I'd warp the tank or blow through with the torch. Everyone says take it to a radiator shop to swap but what tools do they use I don't have? How hot can I go without ruining my new tank?