Excessive pressure or vacuum in the tank is a result of a tank that is vented improperly. Don’t run it again until you’ve resolved. Under pressure you have a bomb; under vacuum (just from fuel leaving the tank with air unable to enter) the tank can collapse - either situation a big safety issue. Air should be able to enter the tank as fuel leaves - usually through a one-way vented gas cap; air can enter, but not exit. If the car has to pass emissions then pressure is vented through a charcoal canister and allowed to enter the intake manifold through the purge valve. Most folks who remove the VATS from ecu's for swaps also routinely disable the purge valve function. I simply routed the line from my charcoal canister into the air inlet pipe UPSTREAM of the maf sensor.

Figure out why your tank isn’t vented properly.

Never owned one - don’t know. But I’m pretty sure for emissions almost everything had one during that time period. Look low up front, sometimes in the front of one of the fender wells.

Question on this. On mine, the MAF sensor will be located in the stock truck air filter box. Can I run the canister vent tube upstream of the filter, or does it have to be between the filter and the MAF? In other words, is there a problem with gas vapors going through the filter?

Also, presumably while filling the tank, it's pushing a lot of gas vapors up into the canister and into the air intake. Is there any detrimental effect of these vapors? Flooding the engine, backfiring, etc?

I wouldn’t think it would be a problem. When filling the tank, the filler neck is open to atmosphere (at least with our gas oumps here) - don’t think you’ll be “pushing vapors” to canister. Worst case for tank vapor pressure is idle when the fuel pump is circulating most of the pumped fuel back to the tank.