49 is exactly right: regular cab long box. The extended cab can be used if you shorten the frame which is easy and undetectable. The rear "C" channel frame transitions into the boxed front section about under the back of the cab. If you carefully cut the factory weld at this junction you can trim the rear frame to give the desired wheelbase, leave a little long to slide back into the front boxed section, and then weld exactly in the same place the factory did. In reality there are so many regular cab long box trucks available you should be able to find one.

Evan