As Joe pointed out, the major insurance carriers offer some exceptional rates for their multi-line customers and I found that I could secure full coverage on my 54 truck with only some minor limitations. This was at a lower rate than the collector car insurers. There are obvious advantages to being able to sit down, face to face with your agent and purchase the specific coverage you want. Getting a full explaination of just what your policy covers, before you need it, just makes good sense.
I can't speak to the service provided by the specialty company insurers because I've never been forced to use them!!! Sometimes the best deals are right under your nose!!!
Dave