Hi Sam,
I think, but could be wrong, that there are two versions of the 4HP22 (which is the gearbox you are after).
I think it depends on the engine management electronics you have, pretty sure that the 4.0 GEMS and later need the EH version of the gearbox - earlier 3.9 and 3.5 engines use the standard version.
But do take care, there are plenty of knackered ones out there - if you can, go to Ashcroft or similar to get one that has been refurbished to some extent.
Good luck, great project.