Keep in mind that you always have the option of slightly rotating the cylinder in order to acquire clearance. You still need to keep it centered on the crosshairs of the firewall, or whatever means necessary used to align the engine. Slightly rotating the cylinder has NO effect on running characteristics. I took this step on a big P-47 to avoid buying an expensive muffler.
So, if you do buy the aftermarket muffler, you have the option of a slightly different orientation to make everything fit.