A good option would be a 4.0/AW4/NP231. Relatively easy swap and with the gearing you have, OD would be nice. There are hundreds of thousands of XJs out there with this powertrain, and can be bought dirt cheap.
Myself, I have a heavy foot and have several Olds 455s waiting for a new home. I suspect that a big engine in a properly geared vehicle can give similar fuel economy as a smaller less powerful engine.
1969 M715 6x6
1963 J300 Swivel frame