little missfire help

Did this "miss" start since you put on the Holley.. Did you set the float level,, if so, try lowering the float level a half turn,, to eliminate the chance of fuel dripping/syphoning down the carb ,, which can cause symptoms as you describe .. then re-adjust later...

Can you look down the carb at the rpm you have probs,, and see fuel occasionally dripping down carb?

I don't think it's timing.. my .02