The 512" RB Duster I had isn't all that comparable to what Feets is doing, but FWIW:
440Source rotating assy, Indy EZ1 heads & single-plane, 750 or 850 Holley depending, Comp hyd roller that idled & ran "bigger" than specs indicated.
TTi headers into 3", 2.5" tailpipes, Milodon 8-quart, FBO/MSD6/F150 coil ignition.
727, 3500 stall, 3.91 & later 3.23 gear, 275/60R15s.
Could bust the street tires pretty much at will, sounded stout, basically pretty hysterical.
Never bothered to check MPG but it was a thirsty thing, guess 8-10 but never got enough highway use for accurate info.

Fueled my lust for a stock-appearing 438-505 low-deck for my Road Runner, I can tell ya that...