If your goal is more legroom, you could simply relocate your seat back further with these:
http://www.ebay.com/itm/1967-1968-and-19...6ce&vxp=mtr

My personal solution to the overall discomfort AND legroom problem of these seats was to put in a pair of black leather seats from a Chrysler Sebring. 6 way power, lumbar support, functional headrest, fantastic comfort...and my original seats sit on a shelf encapsulated in plastic. 8 bolts 15 minutes and I can switch back to 100% original at time of sale, or even for a car show.