I've seen one person claim the catback is of larger sized piping, while multiple others have stated it's the same 3 in and 2.5 dual out.
That would be my question. But I'm guessing that a 3inch in Installer Series with 3 inch out should work with a y to 2.5 if you have dual rears and want to just use the factory bends.
Using the Challenger as my basis, the RT (5.7) models have 2.5" diameter exhaust pipes. The SRT's (6.1/6.4) have the larger 2.75" diameter exhaust. Many of the RT guys will swap their crappy RT manifolds for SRT (or aftermarket) manifolds/shorty headers and mid pipes. The SRT mid pipes (factory or aftermarket) are also 2.75" diameter.
So unless you also swap to an SRT sized (2.75") exhaust, you are faced with needing the 2.75" to 2.5" reducer to connect the mid pipes/cats to the cat-back exhaust. You can use the reducer w/o any ill effects, and many guys do just that.
In fact, many prefer to keep the smaller RT (2.5") exhaust because it builds a little more back pressure. While the larger exhaust will flow more free and provide some higher RPM gains, you lose some low end torque in the process.
All that being said, I think using a reducer on the Ram would work fine. Also, I think having the same size of the stock pipes would be more beneficial for low end torque (towing, WOT, etc).