Kevin,
I can tell you with my 427 stock stroke 2x4s 3.31 rear gear ratio 26.5" tall tire 4 speed top loader highway cruising 75-80 3600 RPM 11 MPG.
The only thing that is different now is that it is 482 cubic inches.
I will report back after this weekend as I am going on a long trip and like to check the mileage just so I know how far I can go between petrol stations.
I originally wanted a five speed just for long trips but was talked out of it by Bill at SA.
The Toploader is a sweet trans but I still would like the overdrive.