www.grundy.com
My dad uses them for all of his restored cars, and they have excellent rates. He's got 6 antique, fully restored cars and trucks, and he's got all of them insured through Grundy.
They have excellent coverage for classics and customs, but they do have certain requirements.
I.E. Can't be driven 100% of the time, you gotta be over 25, etc.
But it's worth a shot. Worst they can say is no.