My experience with speed has been that the exhaust has a profound affect on high speed efficiency. Back when I was running the stock 454, my mileage used to nosedive the faster I went. After adding headers and straight through mufflers, comparing 65 mph to nearly 90 mph, I found the mileage only went 8.5 to 8.2 - not enough to claim any difference. Normally headers and free flowing exhaust does not affect mileage that much, but most comparisons are done at more moderate speeds.
As far as gearing, the 403 does not have much power, so allowing it to turn faster applies more torque to the drive wheels at speed, so throttle response is better. There ends up being very little mileage loss as a result.
Going the other way, what I found was that taller gearing caused the torque converter to slip, so any gain was counteracted by the TC. Once I switched to a 4 spd with a lock up torque converter, then I gained about 1 mpg. All this is with the 502, I'm not sure taller gearing would be practical with a 455 or 454, as it doesn't have enough power to hold speed.
The Flying Fortress
'83 Revcon Prince 31' FWD
502 w/Howell/GM 16197427 ECM/Edelbrock MPFI,Thorley's & Magnaflows,
4L85E 4 speed, KoniFSD,