I have run as much as 22 psi on a T78 on 93 octane(which is very close to a T66) but I think 14 psi as a lower limit and 18 psi as a high one would be safer. You can get away with more, but make sure that you have the car tuned correctly.
Thanks for the reply. Thats kinda the range i was thinking. I was going to set it up to run 18-19psi on 94 pump gas once i get it tuned. That should get me enough power to scoot around rather quickly..