Look at the specs of a gear vendor. I don't think it will live long with even stock cummins torque. A mild cummins can easily pump out 600 foot lbs. I think your are better off with a rear end ratio change with the nv4500 5 speed or nv5600 6 speed. Both tranny's are the same overdrive ratio.
I dunno bout that,I have seen some gearvenders go through hell on the dragstrip. I know there used to be two versions,one for higher hp and torque. I was at the SEMA show about 7 years ago and talked with gear vendors and I do recall them saying they had one available for diesels that made some serious power, but things might have changed since then. This is my plan if I ever get around to putting a cummins in.
It seems to me every time I talk to a guy with a Gearvendor, they are bitching about blowing em up. They seem like a good idea, but for the price of one, you'd be close to putting in an Alison or a built 47RH. Heck, you could find a 6 speed 48RE with the two OD's for dang near the same price I bet.