Running a 4.5 engine with stock transmission/transfer case gears is just stupid.
20hp could be made through bolt on mods. While installing 4/1 gears will be noticed. Then you could install taller gears in your axles or axles with taller gears and get a (slight) mileage increase while having a better craw ratio then stock.
This is from experience through starting with a pos 999 3 speed, installing a Cherokee 4 speed, then installing a 5 speed. Gearing to keep the engine within the power range while making more power through that gearing works.
Having the transfer case set up to make torque when needed works.
Turning your engine into a "stroker" to make next to no gains does not work.