Ok, so it took one day to cut out the old bulkheads and floor stringer under the mast post and remove the other two keel bolts and the boards beneath them. All were soaking wet. I know its a boat, but to enclose wood in fiberglass that will inevitably get waterlogged seems like wasted effort.
The bulkheds I replaced with the same 1/2 inch marine plywood.
But... to improve on the materials used under the mastpost and the blocks for the keel bolts was my goal. I decided on G10 material from McMaster Carr. The G10 is available in sizes that match the thicknesses I needed and, although $50 more expensive than wood, was relativley easy to work with, came very quickly in the mail, and has near zero compression, and near zero moisture retention.
FRP material from the same company does not come in the thicknesses I needed, but it is cheaper.
It took another day to cut the board and glass them in. Really, this is a one-day job if you have all the material on site, but it takes a day to get all the dust out of your respirator and clean the boat.
I took pics of the process and will happily send to anyone who needs them.
FYI, when I put the mast up last week, I had to re-tune the rig to compensate for the 1/2 rise in the mast.