I will take a different stance to the question. Without knowing exactly what Superior did to lengthen the drive shaft, it is next to impossible to know what to ask for. If they used a 2 piece drive shaft, then there is a carrier bearing somewhere to hold up one half of the two pieces. Usually this is mounted at the rear of the front shaft just in front of the universal joint. To make a 2 piece drive shaft, they would have started with the standard drive shaft and then cut down its length, and attached either the original end, or they might have installed a heavier yoke. The yoke is the part that the universal joint mounts in.
Before you take the old one apart, make sure to mark both ends with a indexing line using crayon. Chalk rubs off too easily. Once you have the drive shaft out of the car, then take one half of the universal joint apart. Take both halves of the drive shaft to the auto parts store, and they will measure it and give you the correct part. While you have everything apart, I suggest that you also replace the carrier bearing. Don't forget to put a pan under the transmission tail shaft, since you will be loosing all the transmission fluid out the tail shaft. Now would be a good time to drop the transmission pan and change the filter. You also should look at the front of the front shaft to see if there is a groove cut into the slip yoke as a result of long time use. If there is, they sell stainless steel collars that can be pressed over the slip yoke end to make it as good as new. You will also need to replace the tail shaft end seal. At this point, you are probably scratching your head wondering if you should even attempt this repair. It is a simple thing to do that only takes time, working on your back under the car, with dirt falling into your face. Plastic safety goggles will make this a much more pleasant job. One more thing... once you have it out, replace both end universal joints. They are cheap compared to the effort to get the drive shaft out. Also, I would try to find a shop that specializes in drive-line work. They will have the best assortment of parts and the best quality parts. Get the ones with the grease fittings, and check the box to make sure that they don't say "Made in China" for obvious reasons of quality. Personally, I usually use Spicer parts. Cost more, but they also last longer. Don't forget to make sure to index every part, because if you get the drive shaft out of phase, it will vibrate violently and might even do severe damage to the car.
Your original joints are held in by nylon retainers that will break upon removal. The replacements will be held in with conventional clips. The Pontiac manual has good instructions in Section 4C-1.
Paul
