596 head performance mods

I'm not sure why you feel you need to get additional lift by modifying teh valve arrangement. You can get beyond .650 at the valve without much fancy figuring. In stock port form, these heads will simply not move enough air. I am a fan of larger valves (Tom's heads have 2.05/1.65s 3/8 stem single pce stainless nail heads in them) and I'm not a fan of removing the guide boss or enlarging the bowl areas. Good porting will give great power and more importantly, also give efficiency. What will determine the capability of your heads are the overall job done, including the valve job, which is critical. You are adding a stroke longer than any factory big block. So you will need to feed it. If your heads don't flow huge at big lifts, you need to keep the valve open for a long time in the smaller lifts where they do flow. With a stock port, or one that has been mildly cleaned up but not much else, I would be looking for something in the .570 lift range and at least 260°@.050. That gives you lift at the valve of around .520 and good duration at .300-.450 lift, where the stock heads will flow. The stroke will eat up the duration, especially with a dual plane intake and smaller carb. Do you have power brakes?