Niesel Transmission swap

[mobius]

Hello,
its been a while since I posted on this forum, but I have always found more information here than any other place, manual ect.
I have a 1984 Nissan 720 4x4 pickup truck frame that I installed a turbodiesel from an 83 300d. I installed the MB eng and trans as one unit because they were designed to work together and I really didnt want to deal with trying to adapt something to it. Once the MB powertrain started going in the frame, it was like it was made for it. I will say that it was challenging at times, but it has been the best DD pickup truck I have ever had , pulls great and very dependable when The alt fuel system is taken out of the equation.
I have one slight problem with it, the trans is geared too low for this application. The differential or the donor 300d was somewhere around a 266/1 I think and the differentials on the nissan truck are 3.889/1 so I can go 60 mph, but that is at the top of the comfortable speed, anything faster and Im spitting fuel out the tailpipe.
The transmission is beginning to make noise at approx 320k miles and I know I will have to replace / rebuild it soon so my question is: What transmissions will fit this engine? I would like to find a trans from another MB that might have a higher gear ratio , even if it was a manual trans, I would prefer an auto, but I would do it to see this engine operate at its full potential. As it is now, Im in 4th gear at 40mph. Any suggestions will be appreciated. The numbers off of the engine just below the #5 GP are
617-952-12-042-128 and I dont have the numbers off of the tranny handy, but I can get them. My preference would be to find a 4spd auto from a 300 or 450 sel or some other gas engine MB that would share the bolt pattern and have a slightly higher gear ratio like an OD that is actually over the drive ratio like .85/1. The trans in it now has a 4th of 1/1. Also the gas 300 s and 450 sels are much more common in the junkyards. Thanks in advance, I would have never been able to build the truck in thte first place without this forum.