1995 carb question

[badassformula383lt1]

Ive been going through the truck i recently bought. Since then ive replaced a lot, currently it has a 355 with a cam fuel system consisted of a stock in tank pump leading to a small filter connected to a cheap external fuel pump then going into the stock fuel filter then to a return regulator then to the carb which was a holley 3310-10 750 holley. Ive recently replaced the carb with a edelbrock 1406 600 cfm carb and got rid of the in line cheap external pump because pressure was only at max 3psi. Ive then noticed when i click the key on to start the pressure at the regulator is at 5.5 where it should be but after 10 seconds or less falls to 0 and truck soon dies after. What would cause this?

Would it be the fuel pump or is it something with the computer or wiring only allowing it to prime up but not run consistent?

Please need help asap on this.