Three things come to mind. FIRST, wrong throw-out bearing for your clutch assembly, wrong clutch fork, or you need to purchase an adjustable pivot ball which is located inside the bellhousing and is what the clutch fork pivots on.
Most often, the problem is in the throw-out bearing. It seems the short Chevy throw-out bearing is handed out as a piece that fits and then it does not. There are three various lengths of throw-out bearings. They are determined by their length, - short, medium, and long. Put in the short one, and you will not get the clutch to successfully disengage. You lengthen the pushrod at the clutch fork thinking this will solve the problem, but it improves it but you still cannot get a clean release. Bear in mind that you are creating quite an angle on the clutch fork as fitted on the collar on the throw-out bearing and you can break the collar and if it goes into the pressure plate/clutch at any high speeds - you take out the pressure plate and clutch. Ask me how I know. LOL As I recall, a flat finger diaphragm pressure plate will take a long and the raised finger diaphragm a
medium, or possibly the short, length depending on how raised the fingers are.
There is a "generic" clutch fork sold to fit a host of GM applications to include the Pontiac. It does not. I has an incorrect angle to it and you cannot get enough throw into it to push in the throw-out bearing. This could be a possible problem, or adding to it. The Pontiac parts suppliers usually off the correct Pontiac clutch fork, not the generic one if you have replaced yours with an aftermarket piece.
Third, the factory pivot ball is fixed and cannot be adjusted. Lengthening the pivot ball changes the geometry/fulcrum as it moves the clutch fork closer to the pressure plate. By lengthening the pivot ball, you will get a better throw on the throw-out bearing which will release the clutch as needed.
The clutch fork arm that sticks out of the bellhousing, with the correct throw-out bearing, should be forward of the hole centerline it sticks out of, but not so much as to have the clutch fork arm hitting the opening's edge from being too far forward. If you have the incorrect short throw-out bearing, the arm on the clutch fork will be positioned past the centerline of the opening giving you little space for movement of the clutch fork arm.
Now one last possibility is the wrong cross shaft. You could have a geometry issue when you depress the clutch pedal. If you have a factory piece, you are probably good. if you have an aftermarket piece, it is possible the arms might not be welded in the needed position to give you the leverage throw needed to push in the clutch - but this would be one of the last things I would check.
Read this post, with pictures, as it may help: https://www.gtoforum.com/f154/68-gto-...problem-83201/