Underbed gas tank very slow to fill. Any ideas for venting, etc?

[Old Yeller 1970]

Update:
Changing the angle of the fill line did make a big difference. The next time I filled the truck, which was last week, I was able to insert the fuel nozzle normally and fill the tank with the nozzle on the lowest flow setting. I've never been able to do that before without gas spilling out the inlet.

I tried filling it at full speed but, no surprise, it blew gas back out the inlet. The only consideration about filling on the low setting is when the tank got full it didn't automatically shut off the nozzle and gas blew all over everything. I'll simply have to pay attention to the tank when it's getting close to full and turn it off manually.