Hiya Paul,
Actually, the link given also speaks to a later ignition barrel. The big problem here is that VW changed that ignition barrel almost every year. Further they provided no written documentation (outside of dealers) on those changes for security purposes. The result is that there is very little "hard copy" information out there. As a matter of fact, I and another fellow (a real VW parts nut) could not even find a listing for the ignition barrel (887A) used in the 1970 production year.
But from my own experience I can offer this helpful hint. Take a look at the Special Patrol Group webpage listed above. I'm guessing that ignition barrel will be different than the one shown in (at least) one important aspect. The location of the spring detent hole. On the one shown in the SPG site one has to shape a hook to insert into the hole to trip the spring. You need to find that hole (probably in a different location) on your 68 ignition barrel. Then (pretty much as shown) make a trip wire from a piece of coat hanger. If you press that coat hanger into the hole slowly you can feel it engage the spring and also feel the spring give a little.
Now comes something NOT on the SPG website. Find a dental pick and insert the hooked end into the keyway. Press the coat hanger to depress the spring and at the same time pull out and away on the dental pick. The keyway should come out rather easily (although it may take a little time to get the right "feel" with the coat hanger). The nice part of this method is that it can work for you even with the ignition barrel still mounted the Bus.
Good luck!