In case anyone runs into this same issue, I used a fully threaded 3/8"x3" bolt with 2 nuts and 3 fender washers. You should probably use a 2" long bolt...I had to chop off about an inch since the bolt was really close to the diff.
It sounds just like stock except a little louder. Can't tell with the window up unless you turn the radio off. There is a noticeable increase with the windows down. I dropped off my OEM exhaust at Soul Speed in Austin to do the OEM mod covering the large perforated section. I'm going to try the OEM mod and then decide.
2015 F34 328iX GT Glacier Silver
2011 E90 M3 Monte Carlo Blue
2004 E46 M3 Imola Red