If it's a dually,it's probably a P-30-10,000 lb GVW. I maintained a fleet of them,ranging from 74 to 94. Most likely,unless the truck was built to an oddball spec,it'll have either 4.11 or 4.56 gears. In this part of the country,most of them are either Grumman - Olson or Union City bodies-aluminum construction on steel frame. Places to check for rust are the front fenderwells (particularly under the battery) and the radiator support. These trucks use the same independent front suspension as the C-30-and has the same problem areas-idler arms,inner a-frame bushings and lower ball joints. Check the u-joints in the steering shaft also-even though they aren't hard to hit with a grease gun,they seem to not get lubed like they should be.
Some chassis parts are available through GM-but Workhorse Chassis is the current manufacturer of the bare chassis and you may need to source some stuff from them.
Best thing about these trucks-bodies last forever-wear parts like door latches,etc have been the same for years and easily available online from body manufacturer.