Once you change the cam for power, your mileage will take a hit. A lot depends on the cam size and of course your state of tune.
My own 5.9 is in a B body @ 3600lbs. 727/3/55’s on a 245/60/15. (About 26-1/2 inches) I get about 17 mpg’s Hwy and probably because it revs a bit at speed. (70 mph)
The engine only had typical bolt on parts.
‘72 trap door air cleaner
750 AFB
RPM
Hooker super comp headers @ 1-3/4
2-1/2 exhaust
Fired by a MP chrome box and distributor
P/S, P/B but no A/C.