Hello Carlton,

There are 2 kinds of anti-siphon valve. One actually uses a valve and for this we supply the Scot Valve which we have found relaible in service. The other is not actually a valve at all. It is actually a break in the hose and a small part of the water flow is sent overboard as a tell-tale that the raw water pump is working. When the engine is off this same hose allows air into the exhaust to stop any siphoning effect. Here is a link to a webpage that may explain it better than me
http://www.myboatsgear.com/featured_product/vented%20loops.asp

The size of muffler depends on the length of exhaust. If my memory is correct, you have a Cascade 27 so your exhaust run wont be very long. Follow the basics on the exhaust page of our website. You should not have any back pressure issues using the Beta Marine muffler, a Vernerlift or Vetus waterlock. Work on using a 2" exhaust system and you will be fine. 
http://www.betamarinenc.com/exhaust%20systems08oct.pdf

Regards, Stanley