I would think that the pump/line setup is your issue...A stock tank pickup with a 3/8" line and a mech pump pulling for fuel is asking alot to supply a 500cu.in. motor with a big carb. I would suggest a small electric pump (Like a Holley Blue) to push the fuel up to the mech pump. Install a regulator between the pumps to limit pressure to 6.5-7.0 lbs. This will insure constent fuel supply to the mech. pump..It is better to push the fuel rather than pull it...This will correct you problem and wiring the pump t the ignition will make it as stock as possible. (No switches to remember to turn on).I have seen this problem several times on street/strip cars...poppaj