I dont have any experience in diesel Montero/Pajeros. Well I did get to drive Landraider's SWB diesel conversion and I saw a Gen3 diesel in Mexico. All of the gen2s in the US have coil springs. The v6 in the gen1s is a 3.0 with aluminum heads. Does the 2.5 diesel have a steel head? If so it should weigh the same or may be a little heavier than the v6. I could not tell you if the diesels used different spring rates. I would think the spring rates would be the same. I dont know anything about swapping leaf springs.

I just want to tell you both good luck, and we're all counting on you.