On a car that age, even with the insurance comparison websites in the UK, it is often better and cheaper to go to a specialized broker who specialize in vintage/classic cars. They are often much cheaper and since they are small companies tend not to have online quotations.
The UI really isn't a problem, it just comes down to the company having the coplete set of data, which really is down to them either creating this list themselves or paying someone for it. I'm not sure of the case in the US, but if an online part company like http://www.eurocarparts.com/ or http://www.gsfcarparts.com can do it them I don't see what the problem is.
I rotate through antique/classic cars about every 5 years. I buy one in alright condition, slowly fix it up/restore it on weekends until it's in good enough condition to be a daily driver (normal commute is less than 2 miles), drive it for a while, then sell it and use some of the money to buy the next one. They're really not ready for specialized insurance until the last year or two I own them. That being said...because of the distance I drive, and the fact that it's only used for commuting/events/whatever (no out of town trips, not trips to the store, etc.) I typically have it on the super-cheap "I barely drive it" insurance and the company I'm with now (GEICO) has really good rates for antique/show cars.
That's kind of the scenario I was thinking of for the UK, where they have a very limited mileage insurance (2-3k miles per year). Typically owners of these cars only drive them rarely at the weekends, and only in the summer/in dry conditions.