Eddie - What carb are you running? A friend showed me a dyno sheet where his outfit was running out of fuel, and it turned out to be the needle and seat. I've got the .130 steel needles and seats in mine now and the FE 428 fuel pump and it seems to work OK with the 5/16 stocker fuel pressure line. I also had to use some "smarter" fittings on the fuel pump (found built in restrictions in the brass elbows). But....I think my my horses are down lower than yours - not so thirsty.
It is much easier to restrict pump performance on the suction side than it is on the pressure side. Hydraulic systems usually use a ratio of 2 to 1 (suction line area versus pressure line area). Using the 5/16 OEM line, my outfit works much better with a full tank than when it gets down to a third or less - how full was your tank during the test? If a tank "head pressure" change - changes performance, the line is probablly too small. Going to 3/8 line will increase the crossection of the suction line by about 45% and should lower suction a considerable amount.
Steve Metzger Tucson, Arizona