The KRE combustion chamber is more efficient design than convential Pontiac requiring less lead to light the fire. Typically with say a 6x head a 455 likes 34 vs a 400 at 36(at same compression ratio). I would start at 30 degrees total then try 32 if it likes it you will know. If you gain nothing or lose, head the other direction. Initial varies from car to car depending on compression cam and the starter ability. 12--14 is typical initial. Vac advance is of no consequence at the track but useful for mileage/idle quality purposes depending how its hooked up. Its very application specific but 10-15 degrees additional advvance from vacuum over your total mechanical timing aids mileage and sometimes even engine cooling. Hooking to direct vacuum source (below throttle plate) or timed(ported) vacuum advance vacuum above throttle plate= low or no vacuum adv at idle). Again your engine will "tell" you what it likes. Again dont worry about vac advance for track use. You want 30 degrees total all in before 2500 and if your car likes 16 initial it is going to need the curve shortened to run both. If you cant shorten the curve then do keep the total Mechanical advance at 30 degrees and run vacuum advance to help with idle but I warn you that it wont like it as much as properly curving the distributor.
FYI 2 600's is NOT too much carburetion. Run (2) the same size as you would with a single 4. I have 2 750's on my wifes 406 with no signs of being overcarbureted. Do NOT believe anyone who tells you different. see the following link in the front it shows carb sizing and elsewhere in the same catalog it tells you what I said about sizing, choose as you would a street car.
http://www.exeterautosupply.com/