I had a local performance exhaust shop put a thin wall 3 1/2 inch stainless Magnaflow tip on my stock CBE, just because I wanted it smaller and (here come the flames) "less ricey" looking. It blends into the rear valance and sort of disappears - going for a stealth look. That's what I wanted. Had it done at the same time they installed my DP/RP.
The kind I got slips over the O.D. of the outlet pipe of the muffler at the same place as the factory tip and was then welded in place. I would not want you to rely on my estimate of the pipe O.D. at that point. It is approximately 2.80" at that point, but don't order on that info, please. I'm going by memory.
My point is that you can get a great Magnaflow or Borla tip at just about any good performance muffler shop and have them professionally weld it on for very little money. Pick out the style you like (they are almost limitless) and let the pros do it right, IMHO.