Thanks for the insight. On other cars I've had good success with single 3". More than likely I'll look into that when I get to the SBC swap. I like single because it's easier to deal with on a G-body, it's lighter than duals and you only need one muffler. Last time I did Pumkinator's exhaust I was living in a different area and for a single 3" I paid an exhaust shop $400 for tubing and labor, I had the muffler and dual tips already. Because my son now works for a friend's local hot rod shop we get to use the shop on the weekends so the only cost would be tubing, which is plenty cheap. I already have a couple old sets of mufflers to choose from. His tubing bender only goes up to 2.5" in dies though, hopefully I can talk him into buying a 3" die set. If not I may do 2.5" duals instead.
You might want to look into prefab exhaust sets. With many brands you can buy just the sections you need and supply the rest yourself. One benefit of buying the whole system aftermarket is that you can usually hang the whole system in your garage with, occasionally, some minor fitment issues.
I am running mandrel bent 2 1/2" exhaust with 1 5/8" primary, long tube headers on my factory roller 1996 355 ci sbc.
I find that the duals are a big improvement, especially off the line and on big downshifts.
With respect to single vs duals, even your present 307 would almost certainly run a little quicker (not faster) with duals.