starving for gas

[runamuck]

thought I had a vapor lock problem 63 c20 4x4 on early 70's blazer frame w/327ci bowtie. installed new fuel pump, still runs maybe 15-20 miles and then starves for gas, then installed new red holly electric pump, new canister filter before pump, and a pressure gauge right after a see thru filter near carb, 4 1/2 lbs fuel pressure at carb, then ran for 15-20 miles and same problem, no fuel pressure and see thru filter empty. I see the fuel line coming out of behind seat gas tank is on top? How does this work, is the pickup inside fed by siphon? Seems to me if it's gravity fed to pump, fuel line would come out the bottom of tank, oh yes, I installed the holly pump right under the tank. plus, seeing how it's not the fuel pump, can I run the mechanical pump, with the holly electric still inline, but turned off? (holly is kinda loud)