I've never owned an original Cobra, but I know a lot of people who own them, and I have worked on a few of them. Believe me, most owners of original Cobras are absolutely PARANOID about somebody denting the car just by touching it.
A friend who recently restored his 289 Cobra found that he had dented the sides of his front fenders with his knees while he was installing the headers. Believe me, he was CAREFUL while working on this car. Granted, the "paintless dent removal" guy was able to repair it, without repainting - but, with a glass body, there would have been no knee impressions in the sides of the car to start with.
The aluminium is very esoteric, but I think things are a lot simpler with fiberglass. It's very simple to push around a 'glass car in the shop - push anywhere you like. With an alloy car, you've got to tell your helpers where they can (roll bar and bumpers) and can't (everywhere else) push.
Take a look in your yellow pages, and see how many body shops in your local area advertise that they work with aluminium bodies? If you live in Provo, UT, or Southern CA, you're probably OK. Anywhere else in the country, you have to do a lot of searching to find anybody who works with aluminium. They are out there, but, I expect that shops that are competent with fiberglass probably outnumber them 10 to 1 (count the number of Corvette images in your Yellow Pages bodyshop ads to Cobra images - what? no Cobras? - well then, how about add in Ferrari images too?)
regards,
Jeff