G'day mate,the problem is probably as simple as a hole in the block that hasn't been plugged,as there is 2 holes on the passenger side on the front of the block right next to the fuel pump the top hole should be plugged,if not thats your problem as my camaro did the same thing and nobody could tell me what the problem was,until finally I figured it out by myself.
hope this helps ya out.
Mick