Wow!!! Incredible truck! I'd like to see a build thread on that one! I've seen 1st, 2nd, and 3rd gen rear seats used as solutions for what your doing. I'm sure you know, but the 3rd gen rear seats found on the '03-'07ish quad cabs are slightly smaller than full size, and would accommodate more more leg room. Look up the guy's thread here on the Ultimate Crew Cab...you may get some ideas from him.
And welcome!