[dsraven]

are you saying you will be extending the cab to fit the wheelbase and box length you want? its a bit of work but would give you more leg room. I suggest to add length to the doors and move the posts back. it may look better than a short extension behind the door. it is your truck though so you gotta do what you like.
have you thought about using the donor explorer floor pan and firewall? I sent you a private message in that regard.