454cid
Sooper Pooper
It's all the way custom. GM never made a pickup bed 3500HD (GMT400) - cab & chassis only. The shortest available wheelbase is also 4" longer than a normal longbed pickup wheelbase, so you don't just put a pickup bed on one and roll.. you'd have a noticeable gap. Plus, the frame is different, so bed needs adapting anyway. Etc.
Richard
Around here, those "short" HD's are always dump bodies which makes the price go WAY up, which is one of the reasons I've never been able to get one. The cheaper ones, are always 1/4 mile long and/or look like a raccoon lived in it... and possibly tried driving it. Not that I actually need one.