I Just threw a set of Pypes bombs on mine needs some 12" vibrants bad under there to help with the highway noise and shape the sound a little. They are usually all stainless and unlike glass packs will not need to be replaced after a season or two. (how is 25" going to fit under there if the housing is 4"????)
BTW most resonators if they are listed as "slip fit" are usually the OD size this way you just cut where you need the resonators to go (leaving meat obviously for inlets and outlets so there is something to grab on to) then slip them over the pipes and "weld" them. If you plan on clamping (which I never do but you can) you need a resonator that has an OD/ID configuration so you can run the exhaust into the inlet of the resonator and slip the resonator into the rear pipes then clamp. Otherwise you are talking same diameter pipes and using band clamps.
Welding is the way to go with all stainless steel (304) resonators as they last a long time.
I usually will clamp a muffler, or glass pack but I always weld cats and resonators.