Haggerty or JC Taylor are the most popular, they offer "classic car" insurance that is very economical but has significant restrictions (has to be garaged, drive only to certain events, limited mileage per year, etc.).
If you plan to drive the car often, I would suggest Mercury. They had no trouble insuring my previous TR3A (which would be 50 this year if it wasn't wrecked) or my TR3 (52 this year), on a 'stated' value policy. Basically you state (and pay for) a "not to exceed" amount and then they determine the car's value when you have an actual loss.
I had no complaints about their handling of my claim; but it is definitely "daily driver" type insurance. When the TR got wrecked, they pulled a list of four comparable cars that had sold recently, and valued my car at the average of the four. I could have contested that, but since it was already more than I had 'stated', I didn't bother.