Were you planning on running some sort of inert gas through the tank
while brazing?

I'm told that car exhaust works pretty good for an inert gas. Oil
tankers use their main engines as inert gas generators for the ullage
space above the crude oil level in slack tanks.

Speaking of brazing, I wonder if the torch would cause the JB weld to
flake off?

I'd try putting the whole tank in an oven, and cranking it up in
stages to something on the order of 300 degrees, maybe hotter. Epoxies
soften with heat, plus they have different coefficients of expansion
than filled epoxies. That oughta loosen things up wihout creating a
cloud of toxic smoke.

Then use a brass scraper, and have at it.

===============================================

Have I ever done this? NO. I did get the first idea directly from a
friend who patched car gas tanks. He'd jam the neck of the gas tank
over the exhaust pipe of an idling car, then pop a tennis ball in the
neck as a cork, and start up his torch.

He still walks among us.