If you can weld, you're in business. Pick a muffler and weld it to the extender pipe in the rear, then just clamp the front of the muffler and back pipe, then weld on the hanger to finish.
The Dynomax Super Turbo is slightly louder than stock, but flows great. Worth the $$$, I say. I see them on the shelf at Advance for $30-40.
The Thrush Turbo is similar to the Dynomax Super Turbo, but no lifetime warranty, doesn't flow as well, but is louder. Tough to beat for the price. Advance usually has these on the shelf for $20-30.
Glasspacks tend to sound crappy on anything other than a V8, but the right glasspack can sound ok on a modified 4.0. The cherry bombs and others with louvers inside restrict flow and sound kind of ratty. Thrush glasspacks have a straight-through design without louvers, that flows really well.
You can look to see if any parts stores or exhaust shops have a Flowmaster 40 series or something for under $50, if you like Flowmasters. You could also just call around to see who's got what, if anyone has a muffler for a deal. Offset in/Center out oval ones fit the easiest, but if you find a glasspack or other style, you can probably make it work.