[JohnB]

There is an easier way.
Take an angle grinder and go around the edges of the existing coring. Then take a pry bar, and the top glass will lift right off. Then find a loose spot between the coring and the inner skin, and start working that. I uncored my live well hatch 52x42 in 2 hours yesterday.
Is your coring wood or foam? if it's original, it's eather balsa or plywood. The balsa actually puts up a little more of a fight, but for that, since it is so soft, I just put a big wire wheel on a grinder, and it demolishes it pretty fast. If you skin is already cracked/broken, you just have to be real careful not to make it worse.
When you put it back together, you might want to used epoxy instead of regular glass.
I have a moisture meter, but without seeing it, I would bet it's wet. Almost every one of these old boats has wet coring. I don't care if its a potter or what, they did a poor job of sealing the bottoms of most of these hatches. They were strong, just not water-tight. I doubt they were wondering if it would hold up 30+ years when they built them.