You should be able to put a pair of channel locks against the boss' and turn them with another pair. You can try to get some plumbers puddy on either side of the fiberglass. But depending on the condition of both. I'd dry both sides real well and Silicone the two sides. Make sure you use a liquid thread sealer on the piece that comes out. This will act like a lubricant in order to not bind up those pesky plastic threads.