Not to split hairs, that's Marlin's thing, but I think the 6 spd ZF technically is an RBT?
Frankly, why even do a ring and pinion in your ZF? Those are $3000. Sell your ZF. Then buy the RBT from Loyd at RBT?
You can spec any ratios that you want to. With a 6 speed, you do not need to change out the 4.22.
You could have 5th and 6th as both overdrive.