I am going through the same process of deciding on a muffler configuration for a converted Poulan 46.

I am planning on just brazing the old hole shut (hoping to be-able to braze just with a mapp swirling torch), and then adding in a few pipes. Should I go with anything specific for these pipes (i.e. copper plumbing, electrical conduit etc.)?

I plan on doing this soon any will update with news.

In time, I may try to make something else. I am thinking of going with thin gauge steel(aluminum seems difficult to stick together and not much lighter if light steel is used) and making something simple and efficient. The part I am having the hardest time conceptualizing is how to make the header work out. Solid steel stock as a header flange seems like it would work simply but would perhaps be too heavy. Any ideas on making a nice little header for a simple expansion chamber steel muffler?