As one who is pretty familiar with those boats, and their penchant for rotten bulkheads as the result of
leaks around the chainplates.... here's some
advice...
DO NOT under any circumstances sail the boat. You will have mast in the
water at the first tack in any wind greater than a mild breeze. I know where you are sailing, have sailed it and you will have a disaster.
Get a bunch of jerry cans and tow it gently.
You should secure the chainplates before you take it anywhere, towing or otherwise. Out in the main channel, the wind will create enough motion and stress to pull the chainplates out even if you have no
sails up. Chainplate gives way and your mast is heading for the drink on the next wave. Your idea of running straps under the keel and to the other side is clever....if done properly it might
work well. You might also think of fastening a 3/4" x 6" 48" long piece of
lumber to the outside hull parallel to the deck- hull seam but at least 5" below it using bolts through the hull every 6" and reinforcing with small blocks of
wood on the
interior. Then connect the
shroud to that as a temporary external chainplate. ( you will need to attach multiple lines to the bolts and run them to the
shroud to distribute the stress) It won't be "sailable" but it should hold well enough for gentle motoring. Never underestimate the stresses from a 50' high mast wobbling around in choppy seas. Yes, you will have to do some fill and fix later, but at least the boat will survive.
Be sure to check the other chainplates as well. As you know, they are "hidden" behind some nice but deceiving woodwork. It's worth it to remove that and see what the real condition is. Accessing from the
head does not tell the whole story... check both sides.