Welding it would be the right way. If it was out of the truck you could do a really good job on it with a low nickel 77 rod. You should be able to MIG it on the truck though. Other than that, drill it deeper and helicoil it, or just use a thru-bolt. It's a big enough bolt that you should be able to put enough torque on it for a slip-critical connection so it doesn't jump around anymore. Just don't crack the rest of the flange off.