Couple things as I have actually driven and modded a 1992 ext cab long bed dakota for mileage. Combo was 5.2 518 8 1/4 with 3.55 gears.

I did not do internal engine mods though.

A vinyl roll up soft tonneau cover picked up over 1/2 mpg on the interstate.

Electric fan setup was worth a little.

31"x 10.5x 15 tires helped trim rpms and upped fuel mileage. Better choices than those for mileage purposes out there though.

Mopar performance midlength headers and y pipe with a magnaflow muffler.

Used the yellow Ford injectors as well.

I could knock down 20mpg at 75 to 80mph on I 20 between Atlanta and Birmingham. Made the trip at least once per week for over a year.

As far as compression is it the lack of valve cutouts in the piston tops or the shallowness of the combustion chamber that is limiting you? KB167 would help with the former.

KB piston would also help bump the compression a little with the higher top ring land.