Pulls hard and then quits...
I think it's tuning. Something isn't right with the carb or ignition.
I had an old chevy that wouldn't pull past 3000rpm because the valve springs were so dead.
Again….. It is not the 5/16 line unless it is crushed or plugged.
My 5/16 line fed a solid roller cammed, 8000rpm turning, Holley 800 double pumpin, 340 for 20 years without issue.