you should be able to find flowmaster or magnaflow mufflers that will directly replace the stock mufflers. the stock mufflers should be offset inlet with a center outlet. this would save you some money in pipe and a new y. if you need a resonator to smooth the exhaust note back out and dont want to spend a lot i would recommend a THRUSH glasspack and only thrush because they have a perforated core instead of a louvered core and will not disturb your exhaust flow or hinder it. i think they are $15 - $25 at summit or jegs. if you could post a pic of the catback that you are going to put on i can further help you.
1995 Bonneville SSEi:
(still without a motor and tranny) 15.606 @ 87.95mph (stock)
1998 Regal GS:
DHP 1.0, JMB FWI, ZZP 3" DP, 04 GP 12" Brakes, ZZP 180° Drilled T-Stat, SLP Single Outlet Catback, Silverstars