Several things effect mileage
As dave mentions above.. the carb
1/carbs have a narrow efficient rpm range.. big carb no bottom end efficiency... or torque
2/the number of rpms/ engine volume/ total amount of mixture/ end of the day fuel required per mile... the high stall will be slipping, engine turning over at the slightest load slope accerlation way faster than it needs to
3/ Correct timing AND mixtures at any given rpm and engine load.
4/ cams also have a narrow rpm range, and outside that they become inefficient.. and cruise cams have bottom end torque rather than hp
5/ headers tend to be designed for high rpms and loads , not cruise.
6/ valley cover/ inlet manifold for more economy have longer and narrower runners
7/Fuel type matched to the dynmamic compression of the engine.. cam design.
I don't really wanna spend too much on changing various
Bits and pieces. I can handle 10-12 mpg quite comfortably, just not 4-6 mpg!!!
1/Dump the hi stall.. but keep for a quick swap if heading to the track for a play
2/ do the same with the big carb.. cruiser a 600 vac sec will do the job the carb should give u around a low 15s AFR at cruise and idle and 11.5 to 12.5 WoT.
3/ get the timing in ball park total all in around 32 deg around 3200 rpms,
4/ VA that has around 10 deg in it and is all in at the engine vaccum at highway 60 mph cruise... and still gives around 16/ 18 deg on manifold vaccuum idle degs.
That alone should get u up in the 16/ 18 mpg