I'd bet on a blocked or non-existant tank vent. My old Jeep truck would stall and quit if I ran it on the freeway for long stretches. I ran one of the big Carter electric pumps and 3/8" fuel line and it didn't help. When i pulled that set-up out to go in the rat rod I found the the tank wasn't vented and neither was the gas cap.
I'm fixing that situation as I put these parts into the new ride.
The Jeep gets a new big inch V8 and modern fuel injection, so Lockjaw is getting the hand-me-downs.