Here's what looks like the same model on ebay. Says it's a mercury L60. Have you contacted a locksmith to see if they can make a key for outboard controls...
You could use a fastener that will tie directly to the top of the rib. This is a pic of how I attached my 2x2 braces. They went through the floor into the rib. You could also attach them with two simpson L-brackets like I used to attach the top of the 2x2's in the picture to the underside of...
Not sure about the 56' 10hp but I did a fuel pump conversion with at 58' Johnson 18 hp. I just took off the bypass cover and fuel pump for the 66' 20hp and put it on the 58' 18 hp and never had a problem. Someone here may know if there is an equivalent match for your motor with a 60's model...
You may have some electrolysis taking place. Make sure you don't have any wiring grounded to the boat. I put steelflex on my build and haven't had any issues. https://tinboats.net/forum/viewtopic.php?f=21&t=1745
How about a separate section under boats for the following type posts:
"Should I buy this boat?"
"Is this boat a good deal?"
These seem to clog up the boat section at times. Just a thought. Love the site Jim and thanks for all the hard work and time you put into it.