If your '68 has the original trunk lid on it, do a little creative sanding and see if there are any stripes underneath the top layers of paint. Most re-sprays didn't involve removal of the factory paint or striping, usually just a scuff, prime and shoot. Your blue would have had white stripes if it was indeed a Z, accounting for the white overspray under the cowl vent panel as mentioned previously: wouldn't surprise me as a lot stranger things have happened over the years to our favorite conversation pieces -
Regards,
Steve