Fuel tank help

[GM man]

I've got a 31 gallon 87-91 LMC tank with the four outlet sending unit in my Jimmy. My problem is that when you go to fill up, within a couple of seconds of filling it'll shoot gas back out at you with some serious force. First time it did this, it almost hit a guy standing on the other side of the pump from me! The only way I've been able to put gas in, is to go painfully slow. Even the first notch on the handle is way too fast. The guy that installed my tank was a complete schmuck and it doesn't surprise me that he screwed it up, but is there something specifically I need to do to fix this issue? I'll just be guessing at how to correct it when I get under there. Thanks for any help