Your leaking valve stem seals aren't going to have any significant affect on your mileage. A clogged air filter, worn out spark plugs and tired plug wires are far more likely to be causing your problem.

I guess it just sounds like you want to do an engine swap to try to solve your engine problems when you haven't really put the effort into fixing yours. If you're concerned about fuel mileage, swapping in a V8 is kinda heading in the wrong direction.

95 Montero SR
3.8 MIVEC, Advance headers, 2 1/2" exhaust, Magnaflow muffler, OME shocks & rear springs, 2" body lift, 3" tank lift, 4.90s, TRE front locker, factory rear locker, Roger Brown Rock Sliderz, 315/75r16 (35") tires, Sport big brakes