The 2.5 inch will definitely help you breathe more but I didn't know what kind of mods you've done to your 318 if any.
72bluNblu is right, the 2.5 inch would be much easier because there's "off the shelf" kits that you can order and install if you wanted to go that route. Jegs has a kit and so does Summit.
I actually have the Jeg's 2.5 inch kit that came with the chambered mufflers and it's a bit too obnoxious (loud) for my taste. At and idle it's OK but when on the freeway doing 70 it gets tiring. I can hear it from inside the house when my son fires up the Duster. Granted, my 360 is cammed, big heads, Doug's headers, etc so it helps add to the loudness.
The Super Turbo mufflers that you're looking at, are going to be quieter. Previously, I had Dynomax Turbo Mufflers, but they were 2.25 inches which would be too small for a hopped up 360 like mine. So, I sold the whole exhaust. Actually, I traded it for sail panels.
Keep in mind that the Jeg's and Summit kit do not come with either H pipe or X pipe in case that's any value to you, but they do sell an X pipe kit. If you're not familiar with what that is, just Google it. That's a whole other discussion. lol
At this point, I feel like redoing my exhaust or at least I want to add an H pipe to it. Those kits are kind of one size fit all and requires plenty of test fitting, cutting, cursing, etc but it's the cheapest way to get something working. Get the kit without mufflers and just pick your own muffler and go for it.
