On the top, find a piece of flat stock same width as top cap and same thickness about 14 inches long. Drill it and put your bolts through it. That way you can use some large thick washers that would fit flush over both plates.
On the bottom, drill your holes with a long small drill bit. Then take a hole saw and from inside drill a big enough hole you could get a socket and nut through the hole to install onto the mounting bolt. Then drill the proper hole for mounting bolt. That way you can tighten nut down on solid transom and your not going to really compromise strength of that beam.
Proud Member of Team Geezer
Charlie Weaver USN/ENC 1965-1979