When I built my \6 I found I needed 17 degrees of initial timing and that was with a smaller cam. You are going to need that much or more. Keep advance in small increments while maintaining a constant rpm until you find the peak manifold vacuum, that's where you initial wants to be.
With that kind of initial you are going to need to restrict the mechanical advance, as 1wild&crazyguy stated, or you will have way too much on the top end. Take the point/pickup plate out of the distributor and look at the advance mechanism. The slots in the plate attached to the top shaftis where you need to weld or epoxy to restrict the advance. I use a protractor glued to an old rotor with a piece of coat hanger taped to the distributor housing to measure my progress.
I would shoot for 36-38 degrees total so subtract your initial from that to find the amount you need in the distributor. The protractor will read 1/2 of that (timing is expressed in crank degrees and is twice cam degrees).
One of the springs from the two spring MP kit replacing the heavy spring will get you close on the advance rate.
Don't expect wonders on manifold vacuum, a
[email protected] cam in a \6 is huge and with only 9.2:1 compression you are just not going to have a strong signal.
The 600 cfm carb can be made to work but if the car is primarily a street car you will be happier with something in the 500cfm range. I tried a 600 cfm vacuum secondary and was not happy at all with it and went to a 390 cfm. BTW, don't be confused by the ratings on 2bbl's and 4bbl's A 4bbl is rated at 1/2 the pressure drop of a two barrel so a 2bbl and 4bbl of the same rating the 4bbl will flow almost twice as much.