Hi Tipster, You have a standard mast base casting. I think below the floor you have the plywood/fiberglass/PVC that I described. I would take both of the screws out testing their grip (i.e. the state of the ply) as it comes out. Replace with longer screws as you described. If the wood is not too bad I would fill the holes with thickened epoxy and redrill new pilot holes. If you have lots of clearance at the partners (the hole in the roof) you may have enough room to make new holes a little forward or aft of the old ones. If the wood is pretty bad I would drill the old holes out with a 3/8" bit or larger, fill with thickened epoxy, then redrill the pilot holes. If the wood is really really bad I would think about an access hole or a footwell and replace the plywood.
Sorry, no pics yet, I keep forgetting! Actually, the cuddy interior is on Rev 2 now. Originally, I cut out a hole in the floor just to get my 2 group 27 batteries down low. I enclosed the batteries and there are pics of that interior in my gallery. Last year I cut out the whole area from aft of the mast to the bulkhead. I laid in some wood frames and covered with 4mm marine ply except for the footwell. I moved the batteries up against the bulkhead. It looks nice but there really isn't enough room for humans in there.
They do have a dyneema based line rated for standing rigging. It is a dyneema cover with a dyneema core. But that's not what I used, I used 1/8" 12 strand single braid (Amsteel Blue). I tried it on my forestay first for a year and a half. Now I have dyneema shrouds as well. Dyneema is known for not stretching, however, under condition of constant load for extended duration dyneema will "creep" (stretch). Since the shrouds see a few hundred pounds for 4-8 hours I expected the stretch and put the line under load over time. My original measurement for shrouds was 195" (I use a Ronstan shroud adjuster instead of turnbuckles). The line stretched 6" before becoming stable. This agrees with the spec's I have seen, i.e. 3%. There are several types of elongation spec's (stretch) and it can get pretty confusing, at least for me.
Dyneema has lousy "hand", can't imagine running rigging using it unless it's as a core with a polyester cover. Mmmmmm, light air sheets...