Hey, Willys, sounds to me as if you might be most of the way there, already. I've always had the best luck for a good deep Rrumble, without the rap, with muffs with larger than "normal" cores in them. On my current 350, I'm using 1-5/8 long tube headers, 2-1/2 inch pipes that are about 24 inches long, and 30 inch "packs" with 2-1/2 inch cores, with short "turn downs right out of the muffs; it DO Rrumble. So much so that it sets off alarms on the yuppiemobiles in parking lots. On the big blocks I've had, the same basic theme, only with turbos, instead of "packs"; I don't really understand why, but the straight thru muffs on the big block rapped, no matter the size. Maybe, if I'd used three inch pipes instead of 2-1/2??? Or, maybe, it was because of the tailpipes. But, generally, the bigger pipe gives a lower frequency tone. Just my nickle's worth.

Rrumbler,
LV, NV