Wildcat Fan

I am looking to purchase a stainless exhaust tip (turn down style) for my Dad's 2006 Silverado - 2500, 6.6L Diesel, LS trim, extended cab, standard bed. Since I don't have the truck and I can't ask him to measure the stock exhaust pipe, (it's going to be a gift) I was wondering if someone could weigh in and let me know what inlet/outlet size I should be ordering? According to one website (looking at an MBRP exhaust tip), it says I need a 5 in outlet diameter and a 4 in inlet diameter. The enter stainless tip is 14 in.

Thanks,
Jon