Any backfire/pop back through the carb points to too much initial timing. I'd back it down to about ten to twelve degrees of base timing and see what it does. And yes, if your Holley runs a power valve, and if you backfired it any then the power valve is likely toast, so get your timing set first, see if it feels any better and then replace the power valve and check plug color to see where your mixture's tracking across the range.