I have State Farm - mainly because my family was in the business for so long. My uncle is my agent now and I have my home, life insurance, my family car and both '69 Firebirds with them. No problem on coverage - I have $20,000 on my convertible and I think around $5,000 on my project 350 HO.
Not sure of premiums, but I know its not much. I had the option of having an "antique" plate, which I have, with the normal driving limits. My policies also go towards my "multi-car" discount on my other car.
I have a business associate in the Detroit area that owns a real nice body shop that does quite a bit of repair work on classic/muscle cars for Hagerty with no problems. Hagerty pays him to get it right - no questions. He does top flight body and paint work. His specialty is First Gen Camaro's so if you need body work done and want it done right on your First Gen Firebird, let me know.