Sorry jgmj; can't help you on the '68, but I would bet that somebody in the forum who has a '68 service manual could look it up for you.
The '65 service manual has a table in the first section that lists the dimensions for all the various Chevys, Chevelles, and Chevy II's that year. It's broken down by body styles e.g. 4-door sedan, pickup (El Camino), etc. I would expect that the '68 manual would be in the same or similar format.
I'm guessing that the '68 is bigger than the '65. Non-stock wheels/tires/lowering etc. might change the height a little bit from the published number.
[This message has been edited by JJ'65 (edited 11-09-99).]