I run the elec fuel pump on both my 76 j10 and my 74 Gremlin.
There should be a tank vent line ran with your fill line. It should run from the tank to your filler neck, right inside the nozzle hole. I would check that, since it isn't filling. However, it being an FSJ, have you tried putting the fill nozzle in sideways? Neither my j10 nor especially the Gremlin will fill unless I put the nozzles in at funky angles. And I usually can't squeeze the nozzle handle very much, either. Also check the fill hose and make sure it is running downhill to your tank. I had to jerry-rig my fill hose with an exhaust 90, and I'd have to let it fill, then drain, then fill, then drain. So much better with the right piece now.
When I put the elec fuel pumps on, I remove all the vents (except the one to the filler neck) and charcoal canister, and add a locking vented gas cap. The vent lines from the sending unit, I just get a little piece of hose, and loop them back to each other.
When I get the opportunity (parts) I am going to reinstall charcoal canisters and run the vents. Guess I could use a coffe can, huh? Maybe new project for this weekend
I just saw a great diagram of the fuel tank venting, but can't find it now.
Kinda sounds like the running problem is a little bit of vapor lock, venting should take care of that.