I'd say the '68 mounts will be different. They will probably be a different height, also they could position the motor more forward or backward in the bay. Your best bet would be to just buy some new motor mounts.
If your going TT you would be better off to buy an aluminum FMS driveshaft, the stocker wont take a whole lot of torque.
What is your setup and how much power are you looking to make? I just sold my Vortech S-trim supercharger, my turbo kit should be here next week. I bought a B&G turbo kit with a PT76GTSP turbo. Keeping the boost down to 12psi or so should keep me around the 650rwhp 700rwtq range. Not bad for a street car running pump gas.