My god, how long are your legs! The TR6 has more leg room than most LBC's (emphasize the "Little" in Little British Cars). You really cannot move the rails back without screwing up the way they are mounted, that is, the rails bolt into nuts that are welded to the floor pan. Unless you are willing to reweld new nuts to the bottom of the pan and that would be virtually impossible to do because of the frame rails underneath. You could just do without these nuts and put very large body washers underneath but you will still have to drill through the frame in some area and use very long bolts through the frame, if you understand what I mean. Or, buy a Silverado pickup.
Bill