I use Grundy and they do not (or at least did not when I signed up) differentiate between modified or stock. Their only requirement was that it not be a daily driver, and can only be driven to "events". There are no day/mileage restrictions, and they are very cheap. As far as what the insurers deem as modified, you need to check directly with them. It would suck to insure it as stock and have them deny a claim because they don;t see it that way. Put everything in writing, send it to them, and see what they say. If you don't like it, go somewhere else.