Not a Chevy guy but I know a thing or two about engines, and own a couple 460 Ford power vehicles and my boat has a 454...
Don't bother with that stuff, unless you can set the tune, and even then, it won't make a huge difference on a gasser. Best things you can do if you want more power are to upgrade the heads, or port what you have, up the compression, get headers, ditch the EFI if you can't tune it easily, and throw a carb and regular distributor in there to optimize for your upgrades. My 460 powered lifted F-350 gets 10.8 MPG towing at 70-75 and 12.5 MPG not towing. Has a torque curve like a diesel. Poor MPG is smog engine build, although BBC is far inferior to the 460 there's no reason a BBC can't do similar performance and MPG at that power level.
As for gearing, for automatic there's less reason to go to a lower ratio gearset as you get torque multiplication through the torque converter, to get the engine wher eit makes more torque at a lower MPH than the same truck with a manual. Manual transmissions may need a lower ratio pulling heavy loads since you can't get as much RPM at a given MPH... But every manual I've ever had in a truck has a granny gear so really not necessary if that's the case.