For a larger fuel pump line for above 450 bhp engines, it's possible to replace the late tank sender's fuel line from the small ID stock size to the next size larger (3/8" OD) steel tubing. Above that size you cannot make a tight enough smooth bend in the line to fit under a stock engine screen.
Changing fuel-out line size is a real problem if you have an early fuel tank, as that fuel feed's smallest pinch-point is the welded banjo 'out' fitting on top of the early tank. Bigger pumps run into the fuel-out of the tank restriction problem regardless of size, and are noisy. A temporary 1 gallon tank with an electric pump running directly into the carb should allow you to drive at least 10 miles warmed up in 3rd-4th gear under load to see if you really have a fuel pump, tank or fuel line restriction problem. BE CAREFUL!
I've always had problems separating ignition & fuel problems- the symptoms are the same. I once worked on a fully 'built' 351-C (like yours) that would not rev above 5200 rpms, and we looked at EVERYTHING! A racing shop finally fixed it- by changing the high pressure oil pump to a std oil pump! NO other changes were done and the thing instantly revved to 7000! Still trying to figure this one out- something to do with the oversized pump's load on the distributor drive, but whatever works is right! I do NOT recommend ANYTHING but a std 351-C oil pump on any wet-sump Cleveland.
Good luck.