My neighbor built a '68 GMC regular cab on a 2006 2500 HD chassis.
IIRC, lowering the cab to cover more of the frame wasn't terribly difficult but lowering the bed would've required raising the bed floor if he used a standard bed. (he didn't) I'm not sure how adding the crew cab would affect the ability to lower the cab either.
Anyway, it might help you get some more ideas.
He posted a series of u-tube videos about the conversion.....
Here's part 10
Frankentruck HD