It's also in the AIM and you can download a free PDF copy
HERE.
And, although not for 1968,
THIS 1969 DIAGRAM is fantastic! It puts the stupid symbols in language you can actually understand! And 1969 is very similar to 1968 with only a few exceptions.
There's also
THIS LAMINATED DIAGRAM. I don't have one of these so I can't say how good it is, but I
DO have something similar for my 69 Corvette and it's very good.
EDIT: I just noticed your car has the console and gauges, so you'll also need
THIS BOOKLET.
Ed