OK, now, make sure you know what the policy says. Most "stated amount" policies pay THE LESSER OF actual cash value, or stated value. And in order to get an AGREED VALUE clause, you need an appraisal. It's NOT how much it cost you to restore it, it's a true appraisal.
Now, who's got the best? It's a tossup, between American Collectors and Haggarty Ins. They both do a GREAT classic car policy, but both require that there are no operators under 25, and that every household member has their OWN vehcile to drive, and, of course, that you keep this one in a locked garage, and put less than 2500 miles on it.
To have your 65 Mustang insured up its restored value, most insurance companies will request a current and valid appraisel of the vehicle. To find a low cost insurance provider, Yahoo search "classic car insurance" or something similar, and you will have a selection of insurance companies that specialize in the classic car market. Also some quote sites, if you provide them with minimal information, will farm out your request to several insurance companies who will compete for your business and send you back a proposal offer so they can do the "shopping" for you. Best of luck to you!