402 Big block problem

[Jcentsr6]

I was speaking with one of my techs at work....he is trying to tune my new carb on the truck today. ( I have never had it running great after doing the cam and lifters and all) Well...we hooked up a vac guage and the engine is only pushing out 4-5 psi. I am told that it should be like 13+....Is this correct? The tech thinks we went to big of a cam for the truck. He says inside the carb that the vac is too low to keep some "block" from working properly (sorry I dont know alot about carbs) so it will not run right. QUESTON: What can I do....
Do I have to replace cam again? Thank for your help