Except Luffy has never demonstrated that level of control of his powers. The gum-gum fruit isn't a Logia-type after all, and at least a couple of his moves require that his body isn't prehensile (Gum-gum hammer for instance). I wonder where you got that from.
Word of God has it that he can. Apparently Luffy's bones can shift between states, converting back and forth as Luffy requires. The only real difference between Luffy and Reed Richards is that Luffy can't just stretch as he wills, he needs to put some momentum into it first. (So whereas Richards could just reach up to that top shelf, Luffy would need to wind up as if throwing a softball or punch).
Pilot of the Thing
I'm not evil. My morals just aren't the same as society's.
On a one man quest to beat the Star Wars Universe, using nothing but simple, plain, ordinary logic. Score so far: Me 593 SWU 450