Hey EZ, got your message, will help where I can.
Here's a link that could help.
http://www.sanco-bg.com/NV5600_Conversion_parts_list.htm
Medium duty truck trannies don't have to be originally from behind a cummins as they have SAE transmission belhousings. You just need the correct bell housing adaptor from behind a 5.9 Cummins, flywheel and clutch to hook them up. Easy to find in any truck salvage yard.
Most medium duty stuff is SAE#2 with 1-3/4" input shaft and a 14" clutch. Some medium duty trannies like the allison 545 have an SAE#3 bell which is smaller.
SAE#1 with 2" input is normally what you find on semi trucks.
5 speed, 6 speed and some 7 speed transmissions don't need air to shift them unlike the 8 - 18 speed trannies that need air to operate the splitters making them a better choice for your application.
If you want an overdrive gear in an eaton fuller 6 speed you need to look for the transmission model numbers that start with FSO like the FSO6406 or FSO8406 (Both are 6 speed .78 overdrive trannies rated for 660 and 860 ft lbs torqe respectively). If it just starts with FS like the FS5306A or FS6306A it is a direct drive trannie.
To the best of my knowledge, looking at the gear ratio charts I got from Eaton Fuller, They did not make any 5 speed overdrive transmissions. All are direct drive.
For the eaton 7 speed trannies look for a TX vs just a T......th TX-8607B is a .75 overdrive, the T-8607A and T-8607B are both direct drive.
Now there are some 9 - 13 speed trannies out there with 1-3/4" input shafts and SAE#2 bellhousings if that tickles your fancy.
Now I have not done my swap and may not as I am also considering just slapping the motor in mine and running the wheels off it while I look for a different ride............. BTW, just took a 2003 Freightliner Argosy mid roof cabover for a test ride today.
............ C12 430 hp CAT, fuller 10 speed autoshift, cruise, Jake, tilt wheel, air ride eveything and leather seats too .......