i have a 84 camaro with a carb and am swaping my 305 with a rebuilt 383. the new cam will not allow for the mechanical fuel pump to work so i am going to install an electric fuel pump by the gas tank. do i use the vapor line as the return line for the fuel pump? if anyone has installed an electric fuel pump i would like your input.

Your car should already have a real return line. I haven't installed an electric pump on my Camaro personally, but all you should have to do is install the electric like you said, and I'm assuming you're using a 3-port regulator with return, plumb the return into the existing 5/16" return line (the supply line is 3/8"). Leave the vapor line as a vent to the evap canister.

I almost bought a 350 shortblock (I believe it was an LT1) at a swap meet last year. I decided against it because it didn't have the mechanical pump provision machined. I could kick myself now.

will an intank fuel pump for an 88 or 89 TBI fit in an 84 carb gas tank? and if so will i need a regulator?

Yes to both.