I've run a Nash (Richmond) 5spd on a few street/strip GTOs over the years and I've known a few others who have too. It's a nice trans but, gear the car accordingly. I have a customer who put one one his mildly stock '78 T/A and left in the 3.42 gear. The car went slower with the 5spd and he never used 5th gear at the track. Depending on your combo gear it taller than mornal because of the 3.25 1st gear. A 2.56 through 3.08 may be best. The 6 speed for about another grand can be a waste of money. I know of a guy who runs one in his GTO without benefit. The Pontiac has a flat torque curve and does not need that many gear changes. Now, if you are running "Z" rated tires and want to run at long periods at over 140, then the 6 speed is for you.
Steve
www.pontiacheaven.org