Well if you stick w/the manifolds and want something that "bolts up", you will have to get something from one of the many Vintage Mustang vendors or if you plan on taking it to a muffler shop, just buy the muffs that you want from Jeg's or Summit(you will save a bundle here) and then have a local muff shop fab you up a system. I would stick w/a 2-1/4" system for you current motor setup.
if you are going to do exhaust. why not just put headers on it? save you a lot of time and money in the long run if you do it now, i got hooker headers on mine. cost 150 and then get some good muffs and have someone fab the system on for you.
if you want something loud ( flowmaster 2 chambers) , if you want something that still sounds good and doesnt rock your world, alot of ppl seem to like dynomax( spelling?) mufflers or maybe even something like a Spintech still sounds good. but as said before, if you want it to sound even better think about spending the extra money on some headers, and maybe an X pipe if you want to go that extra mile. and i agree on 2 1/4 pipe for your setup, or 2 1/2 for some growing room if you plan on something more exotic in the cars future.