I recently went through this exercise quite comprehensively when adding the Ultima to the stable. Being able to track any of my cars was of some importance to me. What you will find is that most insurance companies have tightened up and closed the ambiguity and loopholes on Track days and they are now not covered.
I got quotes from Hagerty, Grundy, Sneed, Classic Insurance, Heacock and a few others and virtually all I enquired with have track exclusions. It's right there on the application form in most cases now, if you are an existing customer than I suggest you check teh policy exclusiosn carefully. Grundy were the worst I think the car wasn't even covered at any racing event paddock or car park!!
In general I have to say Heacock were exemplary in their dealings and offered the lowest rates; Hagerty were the most expensive, but their reputation permits some peace of mind factor I guess.
I ended up staying with American Hobbyist (they have just changed to parent company name of American Collectors) and they made me sign an exclusion specific to the Ultima only. Even then it was 'competitive and timed racing events' so permits High Performance Driving Track days.
If you only do very limited track time, you can now purchase a weekend or days track insurance through
www.MotorsportReg.com, underwritten by Lockton Affinity. The cost is around $240 for $30K cover, this is physical damage cover, no liability.