Hi Pepin
When I was sailing an R and bought a new Landy F16 mainsail I had to move the boom/goose neck below the mast rotator bracket but it worked out really well. I had to also adjust the outhaul system to the max length of the boom, that boom was a rectangular shape. If you're looking to change mains and need photo's info on what I did please don't hesitate to contact me.