Depending on the year of the boat there is probably a few hardwood strips that were driven up inside of the crossbar. They will never come out, so getting a wrench in there is impossible. Even if they weren't getting a wrench up in there sounds like a real test of patience.

I would either get a used xbar, or go 1/4" rivets. Use stainless and have someone pull them with a real rivet gun, you'll probably never do it with a hand gun. I carefully mark the rivet holes so that I hit the existing holes exactly. Take your time, drill once measure 4 times.

Sail on!

j