Tapping holes in the lightweight aluminum castings is a really bad plan. There is currently not enough material in the wall of the casting to create an effective thread. All you will do is strip out the threads the first or second time you crank down on the set screws.

Think of what you mentioned about the age of the boats... 40 years. That's 40 years of high stress repetative motion. Even steel will wear out over that time.

Shimming (in my opinion) is the best solution. It allows you to make a custom fit, is easily removed for repair or adjustment, and is easy and quick to do.

We also have to keep in mind the tolerances of the aluminu extrusions and the castings. The tolerance stack up has to allow for assembly of the components when they are at the worst case possible.

Using multiple bolts will only delay the issue. Then you will have more than one elongated hole.

Just to support my statments, I design and build high volume custom automated equipment for a living.