well if it stop at 1300 then you past you idle trans slots or very close?? so it may need more fuel ? hard to think a big block would need that?? and it can have a in take leak to ? if the heads and block was cut the intake will not sit down on the china walls .this will stop it coming all the way down .you some times at the very least cut the bottom of the intake for this .and the intake can leak from the lifter valley to ??????