You should also make sure that the hull core is sealed. If it has balsa coring you probably should dig it out around the opening and seal it with epoxy. If it is solid glass you should at least make sure it is sealed will an epoxy water barrier paint.
I replaced both a speedo and depth thru hull last spring in a 20 year old boat. It had balsa coring and the original manufacturer had sealed it with epoxy paint. I tested it for mositure and it was dry so I just put a couple more coats of Interlux 2000 and then installed the through hulls with proper caulking.