Walker 21471 is the muffler i used. Its for 4” tube.
If you plan to put over 8,000lbs behind an idi frequently i would do 4.10 gears. I feel like we are fooling ourselves trying to pull 10-12k or more with 3.55 gears in these lower hp diesels. If we are going to pull big weight then set it up for pulling the weight.
My big enclosed trailer has really swayed my opinion about pulling. Why work the crap out of it with 3.55’s when 4.10’s would let it work easier.