Those magnaflow mufflers are LOUD. I'm running a 13749 on a 92 318 and a 88 5.9 and the 92 gets me unwanted attention on a 3 inch single exhaust with a eastern catalytic converter and full length pipes.

Comparatively the equivalent walker dynomax 3 inch RV muffler was much better and quieter but was aluminized instead.

I would start by reconsidering your muffler choice.