I plan on rebuilding my 350 chevy nova,and using the 268 Comp Extreme Energy cam and World SportmansII heads my questions are- would this cam effect my power brakes? And since I live at high altittude (colorado)should I go with 1.94/1.50" valves or 2.02/1.60" valves?

I don`t think it`ll effect your power brakes, about 12 inches of vacuum is the limit. Comp cams should say on there site if you`ll have any vacuum problems, as far as I know the sportsman heads come cut for 2.02 1.60 valves.