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You might want to look at running 4.10s... I have 4.56s and 39s on my cummins powered Land-Rover (1993 intercooled 6BT with A518 trans) and it<s about perfect, if I put a 5 speed in it would have 4.10s... These things have more than enough oomph at 150-1800 RPMs.
Now you will be looking for your 10mm metric stuff after working on the Cummins
Edit: Does the GMC have the Eaton or Dana rear axle? If it's an Eaton I want the pig!
I will be running 35's and some of the freeway speeds are 70mph when not pulling a trailer in California. In Arizona and a few other states it is 70mph or 75mph. I think Texas has a daytime and a night time speed limit.
For the RPM to be in the torque band, from my rough calculations I think I need to run 4:56's.
I have the sierra nevada mountains that I will be pulling my 40's, 45's or 60's through. Some of the grades like going from Sacramento to Deer Valley is an 8,200ft climb in elevation in about 60 miles. I want to keep up with the rest of the tow rigs and the rigs that are diving.