Popping through exhaust during deceleration

I'm trying to get a straight answer from someone with experience, this is my first build with this much horsepower. The car pulls great, but when I let off the throttle The car pops pretty loud and even will shoot a little fire out of the muffler. I don't like the popping and can't imagine it's great for this to happen all the time. How is best way to rectify this. My spark plus look like I'm running pretty rich, but I've read that the after firing is actually a sign of a well tuned engine but others say it's too rich and still others say it's lean, any experience/advice is much appreciated.

The engine is a 400 stroked to a 512, about 12.5 to 1 compression, 680 lift comp cam, Indy top end fully ported and a Dominator 1150 reworked by Pro systems.
Thanks again