I wouldn't use a standard insurance company unless you had a custom policy written to protect your investment.
The valuation of the car is all important. If you have saved every receipt from the time you bought the car and paid yourself minimum wage as an unskilled worker for every hour you spent working on the car then you stand a chance at reaching an equitable value for your cars value. Just keep in mind the more it is worth the more you pay in premiums and will have to adhere to the restrictions laid out in the policy (such as where and how it is parked and how much and when you can drive it).
Many classic car policies require a locking garage to store it in (alarmed if it is expensive enough) and won't let you drive on days that have too high a percentage of rain or at night in some instances. They also limit the number of miles you can drive in a year, etc.