All the covers for 311's say they fit up to 31' 6" but I'd like it ti cover the swim platform as well. I think it's right at 33' tip to tail. Should I order one for a 336?

They are 33' including the platform so the 336 cover might be a good idea.

That is how I ordered mine is for a 336. Length wise it fits great. And yes it covers the swim platform.

Do you have any pics of the 336 cover on a 311?