I have actually just had an 80L LPG tank put in my defender 90 2.5 petrol. It was expensive - cost about £1300 to convert (I know it's more expensive than others might have said, but it included free tank of fuel, new spark plugs, 80L tank, kit and fitting, all the guarantees etc. and also a free service at 1000 miles or 1 yr! + I trusted the guy and it was near home!) You'd barely know the difference when driving with LPG and I just run the petrol through the engine every now and then but it starts on LPG. At less than 45p a litre, it doesn't take long to make the conversion worth it, plus the value of your car increases! Look at tank size though - the 80L fits nicely in the 90 going width ways taking about a foot and half length out of boot capacity. The down side is you only get 140 miles out of the tank whereas a bigger one would obviously get you further (I think mine might just be particulary thirsty!)
Hope some of this helps!
