That 427 sound like my buddies cammed 351C with about the same length exhaust. 24" Glasspack "truck mufflers" with a 3 bolt flange right off the 2" header collector in a 72 Mustang. Still sounds like a rotary noise, like a pump cavitating to me but if it don't bother you, run it. Throw some cardboard in front of the rad to heat it up.