Welcome to the 'Bolt! Some explaination is in order. There's no such critter as a 6 lug C 20,all 3/4 and 1 ton GM trucks from '47-current (excluding some '88-'98 light duty models) are 8 lug, full floating axles. There is a 4.10 gear that was used in some automatic equipped trucks fom that era that is a direct bolt in, but it is somewhat rare. Sounds like you have the std 4.56 gear. You mention that you have the 1 ton suspension, I take it that yoou're truck is leaf spring in the rear? That may make swapping in a later diff easier, perhaps one from a late '70s or '80s truck. You couls also swap in a 1/2 ton rear, if that is the way you plan to go, but you should also swap out the front spindles and balljoints to the later 1/2 ton components to keep things matched.
One more question, does your truck have the Eaton style diff (removable center section) or the Spicer/Dana style (integral center section)? If it has the Spicer/Dana, commonly found under GMCs and Camper Special Chevies, that does open up easily available aftermarket gearsets in many ratios, as that axle was used up to current production on many trucks, as well as being a very popular axle with the off road crowd.
