Rusty,
With parts being so rare now have you any ideas about a more modern transmission and transfer case that could be used in the M135?
Yes, I've thought about it a lot. There are several options I believe can work very well.
One being the "AT500" Allison. Note I didn't say the "AT1545" Allison which was used in the M35A3 Deuce.
That one has several problems with the torque converter. The tried and tested AT500 Allison has been used for decades and parts are plentiful. Put a good NP240 transfer case behind it (one that has been upgraded to the "Titan" shafts) and it will last you decades.
I believe I posted my rebuild of the NP240 here somewhere.
The other one is the "1000/2000" series Allison used in the newer Chevys. My 2019 Chevy 3500 has one.
I bought three core 1000's for $300.00 each. I'm actually tearing one down right now.
I already built a NP240 transfer case for this build. I plan on putting this behind my "MultiFuel" in my M35 A2 Deuce.
Now cost is a factor here. The older AT500 will cost around $500.00 in parts, plus the actual transmission.
The 1000 series will cost me around $5,000.00. Thats because I'm using the extremely strong "Input-mid-and output shafts" and also the HyPo C-2 clutch assembly along with the "Raybestos" clutch plates.
I'm also using a three clutch plate torque converter. And of course I will need a "stand-alone" control module. All that costs "bucko bucks" !
A good "stock" 1000 will cost around $1000.00 to rebuild and would work very well in a M135.
The biggest problem will be finding an engine adapter for the GMC 302 . Of course, if you're already going away from stock with the transmission why not just put in a crate 350 Chevy engine.
Now that combination of a 350cid engine along with a AT500 or 1000 series Allison would make the old M135 get up and "dance" !!!!