Outer Limits 4x4 Board

hey guys got a few Q's

got a few series 3 land rovers (all the original 2.25's) at the moment, and been doing some thinking, I use the beasts daily and wanna save some fuel, alot of my driving is up around the 70-80 mark (they don't go much faster ) and do about 125 kays a day,

what would the best way to go?

1. fit a 5 speed box?
2. fit different diffs?
3. track down an overdrive?

also if the 5 speed will make a difference is it possible to fit a w50 celica box to the 2 1/4 motor and keep the original transfer?? (I ask cos I got a w50 lying around)

fitting a 5 speed or an OD doesn't seem to make a big difference to fuel consumption.

The best option would probably be an LPG conversion, which should be almost 100% covered by the rebate.