Personally, I have just a (non-swiveling) cleat on my mainbeam. Used only two large stainless screws to secure it to the beam and it has never failed on me. Using just Parker screws is also what AHPC did for years when fitting a spi halyard cleat. Seems to be sufficient.

I actaully have the cleat on the beam where the traveller rail is as well. The two don't not seem to interfere on my boat.

Wouter