I remember chewing out a depthsounder to replace it and finding that no, the hull really isn't six feet thick, but I don't recall what it actually was. A half inch, an inch...nothing to worry about. Stick a small ruler in your existing thru-hull, see how much of it is actually used and you'll get a good fast idea.
The real question is how the thru-hulls were installed and bedded. Old 5200 can be just like concrete, you may need a spud wrench and Sawzall to "implode" some of the old fittings.