Mike, my '65 did exactly that. The vent tube was blocked completely. If you can get that rig out of there you'll most likely be able to see the problem. I got a new vent tube from Inline Tube. In the meantime, don't fill the tank all the way to full to make sure there's room for the fuel to expand when it gets hot. You'll likely pop something, maybe something else will give out before you get to deal with the problem.