Marty,
I have a copy of the "1968 Ford Catalog Revisions" dated 12/12/1967. Among other things, it states (for the 1968 Mustang):
"Item Revised:
390 cu. in 4V V8 Engine option now available only with GT Equipment Group."
Now this gets tricky, because I distinctly remember seeing a Marti Report recently for a California Special with the S-code engine, without the GT option. So who knows? I am still of the opinion that some things that are so called "set in stone" are open to skepticism. I should have worded my initial reply to this posting differently, to say "supposedly".
This catalog revision is interesting. If you don't have a copy, I'd be happy to fax you one.
Terry