Could someone please tell me the muffler inlet/outlet specs for a 2018 F-150 3.5L EB, I cannot find it anywhere and I have read stuff about adapters. Also, wat muffler would anyone recommend, just the muffler, not the whole system, thank you.

 

2.5 in, 2.75 out. Full specs:

https://www.rockauto.com/en/moreinfo...439151&jsn=450

 

It all depends on what you are using the truck for and how loud you want it to be. Straight through style mufflers like a MagnaFlow, or a glass pack will be the loudest, and flow the best. If loud is what you want, then that or straight piping it would be the way to go.
There are turbo style muffler like the Dynomax Super Turbo or something like that. It will be more mellow, but won't flow as good. Then there are chambered muffler made famous by FlowMaster. Those will have a more metallic or tinny sound. Will be more mellow than straight through design, but a little louder than turbo mufflers.