And he said it originally did have a back seat. It also came with a utility bed from the factory so the dually rear isn't as wide as one that would have came with a regular bed. I'm gonna have to search for pictures to see what he's talking about
GMC vs Chevy is minor; trim and color differences, nothing mechnical.
The utility bed vs a regular bed makes sense as to why he'd keep the old bed, as normal pickup beds are a dime a dozen. (I didn't get why the seller wanted to keep the bed at first.)
That there is what's called a "cab and chassis" truck, because GM sold it as ... well, a cab, and a chassis, i.e. with no bed. Aftermarket companies then put their flat-, stake- or utility bed on it and resell 'em at the dealer.
The C&C dual rear axles are the about same outside width as a standard single-rear wheel axle, so that the tires don't stick out wide like a regular dually.
There are two problems with this in your instance. One, they came in a variety of wheelbases, some different from the standard trucks. This means that a normal pickup bed may be a few or several inches too long or too short to fit centered on the wheel.
It also means that, depending on the tire size and the suspension, that the inside tires might rub on the wheel wells of a regular bed. Since the *outside* tires are tucked farther in, the *inside* ones are REALLY close together.
Now, it's certainly possible to tub out the wheel wells -- I know, I've done it. I don't recommend it, however, as it involves a certain amount of work and you lose the ability to put 48" wide sheets of e.g. plywood or drywall between the wheelwells.
If you are looking for a project that will involve work and some headache -- but might turn out to be something fun -- this truck is it. You could do a flatbed too, that would be cool, or you could do all kinds of custom stuff. Expect some welding, lots of swearing, and lots of measurement.
If you are looking to drop a regular bed on it and go ... well, break out the measuring tape, but be careful.
-- A