extended cab long beds have a longer wheelbase than a crew cab short bed. if you want to put a crew on a extended cab frame you either need to do a 2" body lift to get the cab to clear the bump up in the frame or the second option is to cut the floor and weld in a pie piece to get the proper raise in the floor pan to clear the frame.
as far as wheel base goes you either have to cut the frame down to get a CCSB, cut down a 8 foot bed to the right length, go flatbed, or relocate the springs and axles forward on the frame then bob the back of the frame.
my plans for my ex cab long bed is to cut down the bed, it will end up somewhere around 7 foot.