Everyone who emailed and replied ... thanks for the help.
SO, it seems as though the top is ok. When it's fitted to the car, the chrome part at the bottom in the very back sits almost one inch above the body of the car.
I can't remember seeing this sort of gap on other 280's, but I must admit that I may have not been paying close enough attention.
The car I'm dealing with is a really stunning restoration, but to my rookie eye, it seems as if there's too much space between the top and the body. I can get a pic tomorrow and post it.