Your Spitfire should have eye bolts in the floor already. If not, it almost certainly has at least the threaded holes for same. And a Mk2 should also have a bolt (or, again, at least a threaded and reinforced mount) well up on the rear wheelarch, for the shoulder belt portion, should you so desire.
Oh, if you see a reference to "BS AU 48 1965" on the commission number plate, it means that your car does (or at one time did) have seat belt mounting points complying with that particular British Standard.