I made a flange for my header out of 1/4" aluminum. The Poulan has a sort of D shaped exaust port so it is kind of a pain to mill out.
As for muffler volume Im not sure what would be ideal. There has to be a compromise somewhere. I have been trying to go off the size of what is commercially available for a similar engine size. I think anywhere around 2 1/2" to 3" dia and 10" to 12" long should be close.
I plan on trying some different baffle configurations and even a cannister with no baffles and see how it goes.
Adam