Same thing happened when I filled my '74 when I bought it. Only time I have put gas in it too since I bought it.
I have dual tanks with a single filler neck, so it took forever and I still did not fill them up. I know I need to replace some fuel and vent lines, as some are disconnected/worn. There is also a roll over check valve back there. Maybe clogged or stuck?
I am curious too if there is a better fix then stock.
I've been slowly reworking the previous owners "fixes" and "repairs" and soon will be driving it again.
But since Oregon does not trust people to put their own fuel in their own vehicles, I won't have to be the one standing there fueling my Jeep.
( I bought the Jeep in Washington and fueled it up there)