Matt,
I have the same muffler (albeit with dual 2.25 inch outlets) and I don't have this problem. What's the rest of your setup? Are you running headers (and if so what type)? What's your induction and timing setup? And is the "popping" sound coming from under the hood (carburetor) or the exhaust?
I haven't had this problem. I have long headers and a 4 barrel on an Offenhauser intake. It could be anything from your timing to a leak somewhere. Once your other parameters are known we can eliminate some possibilities.
-magic mike-
