Hi Tom
By 15 mm, I'm assuming that you mean the size of the head of the bolt. The thread is going to be 8 mm coarse thread, which is 1.0 pitch I believe. The Ford hardware catalogue doesn't show the grade for that one, but it may be marked on the head of some of the other pump bolts. 8.8 or 9.8 hardness should be fine.
cheers
Ed N.