To the OP, the difference between the 500 and 600 will be less movement of the pedal to do the same work, your end result will end up being the same. However, the 600 will give you more room when you add cam, head mods, etc.
Street master is a decent manifold, but, since you got the top off.... Have you considered putting a slightly bigger cam in it? 260/260 ISKY/COMP would make a pretty big difference. the 248* .390 lift cam that is in a stock 318 leaves a TON on the table when you go 4bbl.
