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!