Also a good option if you find them.
And on that used carb note - don't try it. Too much of a headache potential.
I'd suggest the new Summit 600cfm carb which has fantastic reviews (and a bowl design that won't leak like a Holley) and no reports of loose throttle shafts out of the box due to poor castings.
However, I think a 600cfm might be a bit over the top for a smog-era 318 with minimal mods. You don't want your engine budget getting sucked through the venturis with no purpose other than to score your cylinder walls prematurely.
-Kurt