What's there to say?
Ultra strong
Bolt behind Dodge/Cummins w/o adapters
Low granny
High overdrive
Can handle anything you would want out of an M35, including vehicle weight concerns. ... The only manual transmission worth considering as a swap IMHO
... Could put a Cummins 5.9 in front of it with little modification
wait wait wait.... you do mean nv4500 and
nv5600, right? because the nv3500 was used in dodge dakotas and jeeps - no, it will not survive behind a cummins. i wouldn't even put one behind a 4bt putting out more than stock torque.
however, i do agree that the '4500 is a great choice for the above reasons stated. and, for those same reasons, it is what i'll be using in my swap.
... Would definitely be a big improvement over the funky spaced & inconsistent shifting of a stock Spicer 5 speed. ...
[edit: are you just saying shifting space or overall fitment?] actually, i don't believe it will [physically] fit that easily - it will take modification. from the initial measuring i've done, it needs to be shortened. the problem is the 2wd versions found in the dodges (which is the one you want to use, anyway) uses a long slip yoke for the rear output and puts the tranny at almost 26" long - too long. i contacted
hi-impact gear and they can put the GM short style fixed yoke output that will give an overall length of about 19" - about 2 inches longer than the spicer, but the shorter cummins should be able to absorb that and more.
the nv5600 (6spd) is even stronger than the '4500, but is entirely too long to swap in without moving the transfer rearward. early versions of the '5600 had issues w/ unexplained random catastrophic failures.
some folks have even had to go as far as swapping in FSO's in place because of reliability issues.
... early versions had tendency to pop out of gear but that's been fixed. ...
are you refering to loosing 5th gear on the '4500? if so, then no, it hasn't been fixed. there are special retaining nuts that must be put on, mainshafts swapped out, or the nut must be tack-welded onto the mainshaft. but any of these mods were not addressed at the factory. the nut and mainshafts are available from several different vendors, hi-impact being one of them.