Thanks very much for the photos, Rick. These give me a good idea of what was there, and should be easy to duplicate.
A couple thoughts for this thread. First, I like the thought of positive flotation in case of two possible calamities, both of which could no doubt be avoided through proper diligence: 1) failure of hose on through-hull connector, for sink [illegal?] and cockpit drain, and 2) failure of repair to hull from where boat was blown off trailer by tornado [true!!]. I sail in two-hundred feet of fresh water, and if something ever did fail, I'd prefer to be able to stick with the boat. No dinghy or life raft, you know!
Second, calculating flotation is pretty straightforward, if not easy. Weigh the foam per cubic foot, subtract from 59.8 lbs, that's the amount of flotation you've got in fresh water (more in saltwater). Multiply by volume of foam. All the blocks are protected from light and weather, so no problem there. Gasoline inside my Venture? I think not! Sure, sealed air chambers would be neater, but we don't have them, so this is the way you hold enough air (or whatever is in the foam) to keep the boat afloat.
Thanks again for everyone's help with this.