Yes, you want a catback, it isn't that much more than just muffler back.

Just straight 40s are sooo loud. Here is a delta 40s i found real quick on a search on youtube of the 4.6L, should give you an idea. I think you will appreciate the lighter drone while on the highway.

YouTube - Crown Vic with Flowmaster Delta 40's Part 2

YouTube - Ford Police Interceptor Flowmaster's

As far as stainless or not, it is just a matter of how many years you want it. In salt country not stainless will last 8-10 years, more in areas without salt.