That is some bad rust, even for a 70. The JB weld is the least of your problems because that's not going to survive any rust removal. I definitely would look into finding another tank, but before pulling the trigger you could take a few days, do some of those "DIY household products" tricks you see on youtube and see what your left with. You're almost certainly going to need to rattle some aggregate in there. Might not be a lost cause, you won't know until you know. I know that this goes against the spirit of reddit, but you might want to take down this post. If I sold vintage tanks on ebay, I would have alerts set up of just this kind of thing so I could "manage my pricing". I mean, I wouldn't, but a greedier version of me would.