The typical auxiliaries came in two basic configurations: one is referred to as a progressive with ratios such as 1.24 low 1:1 direct and 0.86 overdrive. The other has a deep under with 2:1 ratio under, 1:1 direct and maybe a .80 overdrive.
The progressive units are best mated with an SM420, which you have, because you already have a compound low for crawling and you don't need 2x that ratio in low. Aside from that the progressive was designed to split the main box ratios three ways so you have about a 400 RPM drop between gears as opposed to about a 1500RPM drop on the main box that you now have. It makes the truck much more drivable on the road.
Look for tandem-axle trucks. Single axle rigs usually had 2-speed axles. The 5800 series transmissions were often used on gas powered tandem-axle dump trucks. The 6200 series were used on diesels mostly and would work fine on the 2-ton GMC, and the 8000 series were used on big over-the-road tractors behind big cummins engines. They are too big for our purposes.
Whatever you get make sure you get ALL the linkage and the cross member and rubber mounts if any. Get the ends of the driveshafts that hook to the transmission too. The big u-joints and slip-joints are expensive to buy new.
