Generally reading the plugs is the best way to determine carb settings. See Holleytv.com for lots of carb tuning help. I have a 670 cfm as well and I went much leaner on jets than stock. I stayed with silver secondary spring, red pump cam, standard pump jet. Idle screws are way in (1/2 turn up from lightly seated) as well.
Any other too rich symptoms? Black exhaust pipe, backfire, gas smell? Could be a timing issue. I am at 15 idle, 37 overall. It varies based upon the camshaft used not the block. Firing order is also cam determined (HO) not block. Get the engine builder info on the cam if you don't already have it.
It took me about 3 months to sort it all out. Change one thing at a time and keep a log of changes and the results of those changes.
Even when it wasn't running right the Cobra was more fun than a finely tuned BMW.
