Is that still available?
Probably, but if it follows the current pattern, the GM price will be six times the price of exactly the same U-joint made by Spicer (who makes them for GM); I'd price it at a Spicer dealer before going to GM for it.
If it is it will look NOTHING like the one originall installed in 68. Cross shaft will most likely be solid with no depression and have a zerk fitting. I've ordered Precision and AC Delco service replacements and opened both boxes at the counter only to find the EXACT same joint in the box, numbers, stampings, exact. Delco replacement was twice the price.
So nobody knows the GM PN for 68 with M20?