Lets do some math. I currently have 21" between the rear of the cab and the rear tire. If I move the front forward 4.5", the rear goes the same to keep my 117". Now I have 16.5" between. subtract another 12" for the ext cab and now I have 4" between. I could probably cheat 1-2" from the front since the cab isn't as wide and the cab mounts should be different. It will clear, but won't it look weird with the cab that close in the rear. I know, I can cut the cab down so it's the same size as my cab.

I don't know if I'll like the cab being 3" smaller. There's barely enough leg room now. I'd have to sit at a 90 degree angle... I need to sit in a reg cab and see.