RE: wave sail size-ola,kevin?
A VERY difficult question. If you want to "connect" to your 3.3 you can go a lot bigger than 4.2 but thats to small for the 20 knot days. To help you choose we would like to know what the typical wave/B&J conditions are like. "Changing down from slalom at 20 knots" can mean a lot of things. For example, how much "reserve" do you have to hold on to the slalom gear a bit longer, for gusty condition and such things. Moreover, different sails are very differnt when it comes to useable wind range.
What comes to mind is that you should think about a sail like the Hot Superfreak. Its a very unusual feeling sail, but it has a trim range like no other wave sail and is a great B&J sail because its so smooth. A 4.5 superfreak might be a good choice, but you will still get kind of long arms when its howling but the 3.3 is still to small....
As for the mast issue, I strongly recomend keeping the 370 for the 3.3. With a too long and stiff mast, the usuable wind range of the 3.3 will become even smaller.