Dear friend,
Veitnam has equated FJ 40 to a Madhani. Good idea. I think he is right, and you are right in saying that FJ is uncomfortable. However, an FJ is a very good car. It is reliable and will keep going, and going, and going. and It will bring you back from wherever you have taken it.
And you are very right to say that it is not possible to run an FJ on petrol.
A very good option is B3 engine swap, with a 5 Forward gearbox from an LJ. You may also need to swap the axles to ones with Diff Locks. And wider tyres, with relatively mild tread will keep you a bit comfortable. However, to really improve the ride, you may need to improve the leaf springs.
Make it a good ride, and you will enjoy it.....
Best of Luck.
Ranger