I bet. A well taken care of, well tuned engine can run very clean.
It was a fresh build (broken in/rings had tome to seat. Did all the tricks to the carb, tuned it to the best of my ability, dual exhaust (Spintech 2 in inlet/outlet) etcetcetc.
Truck (4x4) pulled over 15mpg highway. Vast improvement because stock that truck new never got over 11 highway downhill with a tailwind the day it rolled off the dealer lot.
So in addition to running very clean on an ‘illegal’ carb, and a ton of no no mods, it also got better mileage.
YAY GOVERNMENT REGS!!!
Oh, and a later build in a heavier Ford F250 with another 400 did similar mileage numbers with head and intake porting, a Boss 351 hydraulic copy and another point of compression...On 37 in. Goodyear Hummer tires and 4.10 gears.