Mactruck
I'm sure you could get an 8' bed on a shortbed truck if you wanted to. The normal bed installed on a extended cab long wheelbase truck is 9' and the normal bed instgalled on a long wheelbase cab and chassis is 11' or 12'. I have a extended cab 3500 with a 10' bed installed. I do have some overhang behind the rear wheels and since I haul heavy oilfield loads, I do have to pay attention to where I place the weight. I got the longer bed because I don't pull a trailer and wanted the most bed space I could get. I've hauled 3000 lbs on the very back of the bed with no problems so it would take a lot to really affect anything. As far as the trapdoor for the gooseneck ball, I have mine in the same spot that they use with the 9' beds. I can pull a gooseneck trailer if it has a longer neck like most of these oilfield goosenecks here in Tx. There's about 3 or 4 inch clearance between the bed and the trailer while turning even crossing ditches, etc so it works ok.
I would assume you would have the same things to think about installing a 8' on a shortbed.
The only time I had any problems utilizing all the bed and overhang was when I loaded a 14' x 5' oilfield rack weighing 6000 lbs. I put the heaviest end toward the truck, but I still think I unloaded the front end by about 2000 lbs because the steering seemed a lot easier.
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1997 3500 club cab, 10' flatbed, 3:54, auto, 4 Firestone air bags, 307,000+ miles