Thanks for all of the advice so far. Jonathan, my hull thickness is 2 1/2", so I could not use the thru-hulls that you have - but thanks anyway. I do understand that the Marelon is a glass-reinfoced product and very sturdy. I just think that if it's going to stay below the waterline, I want the peace-of-mind of bronze. I have enough things keeping me up at night...
As for the thru-hulls being below the waterline, I did some experiments. Before starting this, with 2/3 main tank, and full aux, the top of the thru-hull flange was about 1.5" below the water. Turns out I had about 100lb of lead and other stuff back near the transom, so I removed all of that from the boat. This brought it up about 3/4". I then moved all of that weight to the bow, and that brought it up another 3/4". I then had my wife stand up on the bowsprit, along with the lead (she's about 125), and brought it up so the top of the hole in the thru-hull was just breaking water. That still leaves me with about 2" more to gain before I can get the bottom of the flange out of the water, and there's no more easily moved weight in the transom area left to work with. I have NOT opened the inspection hatch on the bracket yet, but did some calculations: The bracket is about 6-7 cubic feet in volume, which would represent about 350lb+ of water, if full, and all of it is behind the transom. When I haul the boat out, if the bracket is full of water that should give me enough buoyancy to solve the thru-hull problem. There's still the issue of the main tank being 2/3 full, so an additional 30 gallons won't help - but the center of mass for the fuel is about 4' forward of the transom. We'll see, stay tuned.