We did a lot of analysis on this.
I think you need to asses the drivetrain ability to drive reliably.
Questions to ask yourself..
Size of tires?
Gear ratio of transmissions in question?
What amperages will this drivetrain draw when driving? When stalled?
This is when the JVN calculator is your friend.
http://www.chiefdelphi.com/media/papers/3188?
The KOP toughbox mini transmission at best can be 12.75:1 ratio. Not great if you are running 8" tires. (approx. driving speed about 11ft/s)
I think the best options for ease of installation in the KOP frame is one of the ball shifters with the 3rd stage (@64:20)
the overall ratio will be close to 30:1 which will give a good spread of torque and drive speeds.
The only issue you might run into is the VP ball shifters have the motors on the bottom and you will need to accommodate for it if you make a belly pan.
Play with the calculator and verify driving speeds and power consumption.
We decided to use our 3CIM WCP dog shifter running 2 CIMs. We will be adding an additional reduction stage since the lowest gear set isn't quite low enough (about half what we want). We also wanted to use the PTO for our climber..
It will take a bit of work but well worth it for our design.
Good luck with this,
Aloha!