Gotta throw my worthless 2 cents in here. I have insurance through Great American via Midwest Classic Insurance. They insured my unfinished car. Driving restrictions were good... Not to work or school. They have fairly expensive rates though. You can get up to 6k miles, as I recall.
Grundy had one of the most inexpensive rates and unlimited mileage. After my car is painted I will be insuring with them. They will not insure a car under 90% finished, however. Driving restrictions I think were good. You are not supposed to drive it to work or school, other than that, pretty much everything else was acceptable.
Hagertys rates were also quite inexpensive, but their driving limitations suck. Their insurance is for "Trailer Queens" or for vehicles that never see the light of day. Your allowed almost enough miles to drive it up and down your driveway twice. Personally I like to drive my car, so Hagerty is out of the question for me. They were also the most strict about what you do with it.
Best thing to do is call and ask about the specifics because there appears to be vast differences between companies, but from experience, my recommendation would be Grundy.