What camshaft do you have? IF the stock cam then you may have a vacuum leak, 15hg is not high enough for stock. What is your rear axle ratio? What was the benefit to the 302 heads? What year are the 302 heads? If it is not backfiring through the carb when you punch it, then it is probably too rich on mixture, you need to check the float level, idle mixture screw and air bleed, accelerator pump settings. Vacuum advance should be connected to ported vacuum (this means there is not vacuum at idle), correctly connected and operating, vacuum advance will have little to do with hard acceleration. Is your dist a dual diaphragm unit, if it is, one side advances the timing the other retards it, this is for emission? Post your results. Good Luck.