engine exchanges

The easiest swap for a F motor is a 2H diesel which is 6 cyl and doesnt really cost much more 2nd hand.Its not much bigger in capacity approx 4 ltr and the 3B is 3.5+ depending on which version.
The petrol starter motor wont last long on a diesel
Youll need to add a diesel fuel filter and set up the electrical system to power the glow plugs.
Youll need a bigger battery.
The bell housings are interchangeable(I think) but the diesel is made from steel and the petrol is made forom alloy.
Your engine mounts and radiator should be ok.
The input spline are also the same.
If you want to go with the 3B I think you made need a g/box to suit.
I got this info from others and my own observations but please double check all info.

This is a USA site where there are a number of swaps happenig similar to yours

http://forum.ih8mud.com/forumdisplay.php?f=24

If your buying a 40 try and get the latest year possible with disc brakes and power steering and 5 speed .Makes them much easier to live with on a daily basis.
Also the 45 series still has some good ones around and the slightly longer wheel base smooths the bumps out better than a 40 without noticable loss in off road situations.
Also buying a diesel 1st up would be MUCH easier and CHEAPER.
A BJ40/42 or the HJ45/47 are the ones with 6cyl diesels

Also a diesel usually comes with the injector(fuel) pump attached.