The 4JB1T would be my choice, simply because it's mechanical control and stupidly easy to wire in. Glowplugs will be the hardest part of getting it running.

I have a RRC with the 3.9l 4BD1T, but it's a lot harder to fit in than the 2.8.

If you've got the R380 or ZF gearbox the 4JB1T won't be able to break them. The bigger Isuzu's have too much torque (despite the numbers being similar to the rover V8's).
But there's also plenty of room to fit an Isuzu MSB series gearbox in front of the stock LT230 transfer case if you want to make up some adaptors.
I've got the bigger MSA box and yes it's tight.