You will need an extra long through hull. Maybe that is why that one is mounted the way it is. Are all of them mounted the same way? Are extra long through hull fittings available?

I do not through bolt my flanged Marelons.

Here is what I do:

Cut two pieces of plywood - one 1/2" or 3/4" thick and one 3/8" thick.

The thick one gets a hole the size of the pipe, the thin one gets a hole large enough to fit around the nut.

Contour the thick one to the hull.

Paint both with either epoxy or a good marine paint like Easypoxy.

Sand the hull area under the backing block until it is clean and there is no loose paint. If in doubt about the paint adhesion remove the paint to bare fiberglass.

Install the through hull with a good caulk (I prefer 5200 for this) and apply the caulk under the plywood to fill minor differences and glue it to the hull, then secure it with the nut.

Let the caulk set up - wetting it accelerates the cure.

Put caulk or teflon tape on the pipe threads, put on the thin plywood and screw on the seacock.

Drill small pilot holes for screws and screw through both pieces of plywood.

5200 is an excellent adhesive and combined with the nut will hold it all together forever.

Sit back, have a beer and congratulate yourself for a job well done.

If you use this technique, using the nut and use Marelon seacocks and through hulls you can actually change a seacock without hauling the boat.

Plug the through hull with a piece of cloth wrapped in a plastic bag. Unscrew the seacock and put on a new one. Place a dowel rod in the closed seacock hold a rag around the dowel rod and the seacock. Open the seacock and push out the plug. Pull the dowel rod up and hold the rag to block water flow. Close seacock. I would not try this with bronze - they tend to weld themselves together and a weak through hull might snap.