The only gain the dually has is more towing capacity
I have heard the opposite. The D70 has a weaker pinion design. Granted when hooked up to a 6.2 diesel I don't think there is much worry about destroying an axle due to too much torque.
D70's were used in 1-ton dually GM pickups (normal pickup w/the wide fenders), the 14 bolts were used in a dually configuration in cab and chassis trucks, and as single wheel configuration in just about every 1 ton (and some 3/4 tons) GM Van/truck, so they are more than plentiful.
To Matt Kahle; were you intending to run dual rear wheels? Dual rears are not as good as single wheels for offroading, especially in snow and mud. There's more tire to push through mud/snow. And duallies do help distribute the load, and increase the capacity of trucks, but I don't think it would make a big difference in a 1009...maybe give it a little more stability.
The wheelbase is too short, so that is the biggest limiting factor of a 1009, and you can't do anything to change that. The next biggest limiting factor of a 1009 is the gear ratio...its too high to tow a lot with, but you can change that.
How much weight do you plan on hauling? And in what type of trailer and hitch set-up?
A civilian K5's towing capacity is listed at 5000lbs with 450lbs tongue weight (most civilian K5's had lower gear ratios). Probably wouldn't be a whole lot of fun pulling that much especially if it isn't loaded properly. With the D60/14B (with their lower gears), and good brakes; it should be able to comfortably pull 3500-4500lbs on a trailer with trailer brakes, a load distributing hitch would help a lot too. However, if you get D60/14B 4.56:1 gears (standard in 1008/1028's) then you lose top speed...limited to about 60mph.
To summarize...swapping in a D60 front and 14 bolt full-floater rear with a locker, 4.10 gears, would be a great mod for a 1009 that's, looking to do some medium hauling and solid offroad performance.
But if you need to consistently tow over 5000lbs, I would look for a different truck
