Carriage bolts, yes. Backing for the heads- I'm hearing none needed. I thought of brass/bronze squares (done nicely).
I've got no idea about the thoughts and intentions David; what I do know is I wish Irwin had done these 'normally'.
Yes if Irwin had made this easier it would be better. Its too bad they didn't think it through.
The idea of going external creates several problems, in my mind. Every job is a compromise so none of these problems may affect you.
The following are guaranteed:
The new chain plate will have to be bent to follow the curve of the hull and then bedded.
The new chain plate will exit the topsides differently and the toe rail will have to be fixed.
The new chain plate will have external bolt heads that will not be the original look.
The following are just my paranoia but might happen:
The bolt heads may distort the glass or gelcoat
There may be hull distortion due to point loading.
Someone looking to buy your boat may see a non standard repair and be put off from buying the boat.
The option of putting it back together the way it is with optionally machine screwing some of it together to facilitate removal for inspection seems safer and neater to me and when you add up everything maybe not much more work?
Please don't take my comments the wrong way. Your boat, your decision. You have lots of more experienced folks than I advising you. Just putting my internet 2 cents in which is worth probable much less.