"and cut an aluminum disk to fit between the steel post and the wooden post that you see in the head. "
Aluminum plate, or unknown alloy, next to steel? If you don't insulate the two (i.e. put in a piece of UHDPE) you are likely to get galvanic corrosion there in a mainre environment.
I've seen Bob Perry's drawings for the I28 and they show several differences from the production boat. One of them is what appears to be a sensible keel-stepped mast instead of the offset kludge that actually got built. But, of course, if they'd built it with a simple mast they couldn't have shown the pictures of the "huge" open interior, which probably sold more boats for them than anyone asking about how well plywood would age. But at least for the deck and cabin top core, they did use Bruneel plywood, which ages like iron.