First off, Dont flare it at all unless you plan on running a wheel/tire combo that will actually make use of the extra room that flares give you. otherwise, there is no point.There are too few 510's around, especially ones as nice as yours looks, to be doing unnecessary torture to the body panels while attempting to make it look like a racecar.In my opinion box flares are ugly, bubble flares (like on the front of your car) are not great looking either. If you can stick with stock size fenders, I think you would be happier in the long run.
When you ask about rolling, maybe we are thinking of different things....I have rolled all the rear fenders on the 510's I have owned in order to get descent tires under there. There is a lip under there that you have to slice and bend upward so it doesnt rub on the tire. Usually when I roll the rears, you cannot even tell that anything was done, it doesnt look pushed out all. You will not be able to roll it enough to make it looked "flared" in the rear.
IF you decide to mold-in some fibergalss rear flares, then god help you. They are very hard to do correctly, and will most likely end up in a dissaster of a rear quarter panel. In the old days, when these cars were prepped for the Trans Am racing, they would flare them with hand-sculpted metal flares, truely works of art.Just my opinion, leave the stock fenders on there and get an engine in that thing.