i want to use my 2 1/2" ultraflow mufflers but want to run at least 18"long collector extensions from my 3" o.d. collectors.i dont want a crossover pipe,just true duals.if i place the mufflers as close to the rear of the car as possible will that help to smooth the transition from 3"collectors to 2 1/2" mufflers?

Flowmaster makes some very nice long 3" to 2.5" transitions. They are about 10" long and have a gentle reduction in size. So you could use your 18" extensions off the collector, followed by the Flowmaster transitions (reducers) to the 2.5" pipes to the mufflers.
I use their 3.5" to 3" parts in my 3" system with an X pipe to 3" DR. Gas Spintrap muffs with 3" tail pipes on my Nova with excellent results. No HP loss as near as I can tell and it will pass the 88dB limit at our late night drags.
BTW.....I really think you shold consider an X pipe for your 455. They flat work annd sound good to boot.
Jegs makes their own for cheap............
Ron

thanks Ron454,i'm definitely going to look those up!