...I typically tap the fiberglass first. I read in one of Ference Mate's books that Hinckley did this and it made sense. It is easy to do, I just drill pilot holes of the correct size and then simply run the tap and thread it afterwords.
The next step is probably the most important (in my mind anyway) and that is to bevel the hole at the top between the fiberglass and the item being bedded. This leaves room for your favorite caulking material. I simply tighten everything down from the fitting end, install the backing plate, washers and nuts and clean up whatever caulk has squeezed out. Nothing bedded this way has leaked for me yet!
If I am simply re bedding an existing item, I make sure the hole is beveled and I hold the fastener from turning as much as possible and tighten from the nut end thus avoiding the shearing/ wiping away of sealant.
This has worked well too...