As far as I know, the GM90's have freeze plugs at the TOP of the head...like in the oil galley area. I've had one come out before. I was easily able to find a replacement from NAPA.
Replacement is simple, but I DO use 3M Super Weatherstrip Adhesive to "help" it stay in there. Just find the appropriate socket that'll "back up" that disk, and tap it in place, with some super weatherstrip adhesive on the back side....just to make sure it doesn't leak.