383 stroker

I just installed a new GM Performance SP383 stroker in my street rod. I don't have many miles on it yet and have been taking it easy to break it in. I noticed a slight backfire on hard accel (tried a couple). All the vacuum connections look fine. I am running 91 octane (spec sheet says to use 92 but none of that is close to me). I am also using an Edelbrock 650 cfm carb from my old engine...spec sheet suggests the 770 cfm Holley. I haven't checked the timing but, at this point, I trust that the mechanic set it up per specs when he installed it? The vacuum advance is connected...spec says not to but not sure that would make it backfire? Any thoughts guys? Thanks

put dielectric grease on the spark plugs

What kind of fuel delivery are you using, carburetor, fuel injection, etc.... Also, what rocker arms... I would certainly look into the timing first.