My C34 is probably close enough to your 350 for me to weigh in on the subject. The mainsheet clam cleat is made of metal and is the largest of its kind made (I have the specs back home). Unfortunately, the 9/16 mainsheet is on the upper end of the line spec for the cleat, and if it is not properly set, it will release. The cleat is lower than the fairlead from the deck organizer to the winch so you have to remove the line from the winch to get a good bite with the cleat. I do this by pressing down on the sheet as Iím releasing it from the winch. I will often give it a tug after this to make sure the line is set in the cleat.
You do not have many alternatives. Catalina embeds a bronze plate in the coach roof and the cleat is drilled and tapped into it. There are no clutches made that fit the hole pattern (Iíve checked). Even so, a clutch is inadvisable. I know, Beneteau does it. But if you ever had a mainsheet hockle in a clutch, you will regret ever mounting the clutch (Iíve broached a Beneteau First 38 with just this same occurrence, not fun. And if you ever do it, donít have your wife on board!). Cam cleats are also not advisable as you need to pull on the line in order to release it. So again, with the forces on the mainsheet when you are most likely to blow the sheet, you again will be stuck. The best alternative is to get rid of the NER yacht braid mainsheet and replace it with something made of endura, dyneema, technora or the like. This will give you a smaller diameter line for the same strength and stretch properties and it will fit better in the existing cleat. The added bonus is it will also fit better on the ST30 winch.