Is this the insurance co. of the driver that hit the coupe? or your own? If it's the former, and they try to lowball you, you could sue them for the true replacement value. Most people wont go to this extreme because it cost money for the lawyers, so by the time they do get paid, the lawyer gets most of it. The Insurance Co's know this so they will ALWAYS try to pay totaled claims at the lowest cost possible. The only time they can't get away with this is when classic car owners carry their own stated value or agreed value policies and file claim under thier own policy. Then the owners Ins. Co. goes after the other Ins. Co. and the other Ins. Co. ends up paying something ALLOT closer to the owners replacement and or agreed value cost. You think Haggerty pays out claims without going after the other guys insurance co? I think NOT!
My suggestion would be to meet with the appraiser when they look at the car and ask them questions about weather they've done many Classic Car appraisals and on what make/models to see if they have experience with Classic Mustangs so you can get an idea as to their experience/qualifications. If the appraiser hasn't done old cars before, you could still protest whatever value they come up with on the grounds the appraiser isn't experienced and/or qualified to appraise your car and request/demand that an appraisal be done by a "Qualified" third party. Maybe also come to the appraisal armed with the NADA book and some Mustang and/or Car Trader Ads of similar cars. If you've got resale comparison information with you, you might be able to discuss/argue the value with the appraiser before they submit it to the insurance carrier.
In the end, it all boils down to $$$$$$$$$ and weather or not you keep the car for salvage
Just my $.02