You might do even better if you were able to regear the axles (though I know that is a lot of work and expense). That is pretty tall gearing especially with the bigger tires and size/weight of the truck.
To give you an idea, in the Volvo 7 series TD cars with factory automatic, the tire size is 195/60R15 and the rear end is a 3.91. The torque converter lockup occurs at about 45mph and at 70mph I think the revs are over 3000. So, much shorter gearing and smaller tire circumference, plus of course it's a much lighter and smaller vehicle, and only driving one pair of tires.
IIRC the Land Rover transfer case runs 4x4 full time with a center diff right? So more driveline friction and inertia too.
Seems like in your setup a set of 4.30 or 4.56 gears in the axles might be about ideal? If such are available?
Or just run it in low range all the time....
Would be great to see some pictures or video of your installation sometime, I would like to try one of these builds myself someday.