One reliable source of potential value can be Auction Results from Mecum Auctions and similar. Only issue my be the low numbers such cars have shown up at those auctions. Might have to go back 5 years to get enough of a variety of cars to consider.
A local business owner was insuring an AMX his father had ordered new. So Chris put what he suspected was a normal value, then factored it up a few thousand USD for general principles. He submitted the application to Hagerty. A while later, he got a "rejection letter". He questioned that until he read the letter! Rather than the price he had put on the car for its value, THEY suggested it needed to be higher! Why? It was ordered with the special HP performance option, which was a very rare option for an AMX. So their recommendation was in the $50s rather than $30s, in USD. THAT told him something he did not know about the car, or that he remembered about it. So he sent them a check for THEIR amount and he was happy!
I would suggest the "agreed value" coverage option. Just update it every few years.
Haggerty has an online value guide, too. Might look at it.
Just some thoughts,
CBODY67