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.
