Good luck with that. Steel thread on cans vs brass threads on heater when cross-threaded = stripped mess.
I know of no way to recut inner threads on a big piece like that, and I've done a lot of rethreading/cutting new threads with tap and dies. Maybe you could find a Helicoil that big; I dunno if that would work on there. Best thing in the future is what I've done: I always put a drop of lube on those threads, or on the can if they look rusty, and screw it on while tipped on side to be able to see that it's going in straight. Good luck.