I read your post, and understood completely. I'm familiar with Old Cars price guides, and realize they aren't the only valuation tool available.
Hagerty's valuation was specifically for a '68 383 coupe. Add ten percent for 4-speed. Hagerty is in the business of buying these cars when damaged. I'd imagine they have an idea of their value.
My intention was to reply to the OP. I did so, and don't intend to go down this rabbit hole with you debating which valuation method is the more accurate.
Have a nice day.