As far as I know te best you will get is a 3.9 out of a KE50 import. Rocking horse poo if you can get it. What's in it at the moment? Belive it or mot, going from 4.3 to 4.1 makes a fair bit of difference, the only thing is, you'll still be doing 3500 rpm at an indicated 100 because you've changed the final drive. The trick is knowing where 100 actually is.
Or you could put on bigger tyres.