Somebody here has pictures of the hitch set up that will let you pull bigger loads.Don -
Are you thinking of the Trailer Toad? http://www.trailertoad.com/index.html
IMHO, these are cheap insurance if you're going to be pulling a heavy trailer with a GMC. Takes the majority of the load off those two bolts holding in the cradle. Not a bad idea for other chassis owners, either.
It's not so much the static tongue load that's the problem, it's the dynamic loads induced while bouncing down the super slab. Just run alongside somebody towing a heavy trailer and watch what's happening at the hitch point - it will really open your eyes!
A good hitch on a GMC will have stringers going to the rear radius rod attachment points, that way the horizontal loads are tied into the chassis, not the back bumper - again, taking load off those two bolts.
I've seen a couple of GMs buckle the roof above the rear axle from towing heavy trailers. . .
Tom's point about towing with a stick shift applies to ANY 4-spd manual bus, not just GMCs. Of course, you have to pay closer attention to your temp gauges when towing with the Allison, especially in unlocked converter mode. Best is to avoid getting yourself into that situation in the first place, regardless of transmission type.
FWIW & HTH. . .