I had to bring this back from the dead because when I was helping my mom clean out some old buildings on the farm that are coming down soon I found the stock muffler from my car that I removed the summer of '87.
It measures right at 32 deg. starting 5/8" down from the top.
The end is flat at the top, then angles from there.
Measure down 5/8", then start your 32 deg. angle cut.
This info is more than likely too late for the OP, but hopefully it will help someone down the line.