I've successfully used JB Weld to seal some leaking seams on a gas tank on a 48 TE20, the tank mounted over the engine, and it held leak free for 16 yrs and is still probably holding.
In this case, a joint thats prone to moving, I wouldn't recommend JB Weld.
Radiator/tank repair shops routinely cut open tanks to clean them out then resolder them back up, I'd run the tank down (or drain it down) pull it and let a radiator shop look at it, as stated your looking at an hr or so of labor, much cheaper than a new tank.
Good luck
Dave
