Unless the car is new or a late model, there is no retail value as the car is a one of a kind. Blue book or retail value is based on year/make/mileage of a non modified car. So this cars value for insurance purposes would be what it would cost to do build an exact duplicate of this car if stolen or destroyed. Now a retail market value will be different as it is based on desirability in todays market. It is worth whatever someone will give for it. So, for insurance you will need to go to a collector car insurance company and basically they will insure the car for whatever you both agree on, since the premiums will be based on the insured value.