Look at the stockers in the lower section of this chart. 21848 is the HEMI sized muffler. The ones from Waldron and others will flow better but this data will give you an idea of the stock replacements and how they react.

Note something else: it is easy to see why the old Corvair turbo muffler worked, in fact, they flow about the same as today's leading chambered muffler with significantly less sound levels.