I'm with Mark....split the shifter. You really won't use it much, at least on dry land, but in the water it is fantastic. As far as the added stress of the 8 wheels, the Avenger only has 6 touching, the bands will just slip if it does take too much, and there are tricks to making counter rotation easier. Mark had his fancy sophisticated terms like "static and sliding".....What I know from playing with it, scratching my head, and playing some more is: It IS tough to counter rotate from a standing stop...I'm guessing it is from trying to slide 2 tires each side sideways (4 tires) at the same instant. But if you start moving (just creeping) counter rotation is much easier. Where you really notice the difference is on hard gravel or concrete. Loose/soft dirt or mud counter rotation is easy. And that is with a 16 hp briggs in the tan MaxII I have. With the bigfoot (40 hp) it doesn't matter.
Going over the top of the t-20 with the linkage would work, but would make the sticks backwards....unless you built a simple bracket with a pivot. You could even bolt that to the top of the t-20.
Mike: Dibs on the Kohler if you decide to sell it
I wish I was closer...I'd come over and volunteer some labor.