I'm going to guess, based on only what I've seen, that most of the 68's were autos. The Turbo 400 was a fairly new option on the GTO and was very popular. They came with the Hurst dual gate shifter, which made the automatic pretty decent. With stock tires and a shift kit, you had your hands full if you nailed it at about 30 mph. It was hard to keep it from going about 30 degrees sideways. I had a 68 with the 400 HO motor, which came with the same cam as the standard motor, but had the good exhaust manifolds . On the manuals, you got a 10 degree bigger cam and the manifolds. Mine had a 3:42 Posi and got the bigger cam about a week after delivery. I loved the car, but it left my life for a series of Trans Ams.
I understnd your frustration with the sunroof, but they were really popular additions in the 80's. There are no specific problems with the 68s. They can have the usual rust problems, especially around the rear window where the water puddles under the molding. I think the 68s and 69's were the best looking of the series. The lines were clean and smooth.