If you are talking about bungs in the exhaust....you absolutely do not need them if you are deleteing. As far as 4 or 5"...it's a personal choice...maybe do a search and see what folks think. Some like the 5" for better sound. I have a 4" system and I'm thoroughly satisfied. I had a 4" system on my 2002 D-Max...and was extremely happy with that. There are you tube videos out there where you can listen to the sound of each. Again, it's a personal choice...either one would be an upgrade over the DPF/SCT.