I need some help on a muffler for my 3.5 K&B outboard. What is the best tuned muffler for that motor?

How are you going to connect it? Are going to cut out part of the lower unit?

He's probably got one of the models that comes cutout already.

Mosh, I only run the stock outboards, so I've ignored what's happening with pipes. Try one of these places, they will be able to help you:
http://hyperformanceproducts.com/
http://www.grimracer.com/