the car it is in, the 318 block or the 273 intake?
ok, so you've been given a few different things to check out first
did you clean the idle circuits and and what are you using to set your idle speed...the screw or the throttle cable?
what kind of transmission does it have?
and if it has an automatic, what is your idle RPM in neutral, park and drive?
any chance you did a compression test while building it?
i guessing this is a ground up build, no?