As I mentioned I have a 3600. The original HO52 axle has a 4.56 ratio (some are actually a higher number) that I didn't really want so did a 14-bolt axle swap to get to a 4.10 ratio. I don't know if the 14-bolt brakes, etc. would work with the HO but I have my doubts.
The output of the transmission is an enclosed type drive shaft to a mid-ship bearing and an open drive shaft from there to the rear axle. I don't know if the overdrive that you mention can be adapted to the output of our type of transmission but maybe with some additional transmission changes to a more typical SM420 it may be possible. I am familiar with Novak and always liked their products although a bit pricy. However, I am not sure there is room for an overdrive unit at the back of the transmission without running into the frame cross member that supports the mid-ship bearing. The fornt portion of the drive shaft is fairly short.
I'm not trying to diswade you just trying to let you know what you may need to know as part of your decision process.
Thanks for the info. I don't know a single thing about torque-tube driveshafts, so I'll have to look into that part.
I do have a SM420 output yoke cover (I'm not sure if I''m using the correct terminology) and a "mechanics style" yoke from the SM420 I put in my Jeep. The transmission was a NOS, early-'80s-built unit made by ConDiesel for the M561 Goat but Novak's SM420 to D18 adapter fit without issue so I'd imagine and hope that I could slap a conventional (modern style) yoke onto the back of the SM420 that's in the truck.
As is obvious, I'm unfamiliar with the underside (or any side really) of these trucks. I'll probably just have to wait until I have a truck and see how much space and flexibility I have and how much custom fab work a swap would entail. I built a crossmember from scratch when I did a SM420 swap on my '66 Jeep and it hasn't broken yet so I'm not too worried about that sort of fabrication work, but every little mini-project added does play a factor in the overall difficulty of adding an overdrive.
