flow and sound better

Mikes advice is good. His opinion on mufflers is actually correct. The FlowMasters do not flow as well as other mufflers. A deep rumble sound comes from chambered mufflers. Not glass packs or Cherry Bombs.
Dyno Max mufflers have a great rumble. I also like the Hooker Aero Chambers.

If your going to shoot for some decent street HP, over 300 HP, I would go right to 2-1/2 inch on ethier engine. Just choose an engine. And IMO, if you have a 340, go 340.

I used Hooker Comp. headers with a Jegs exhaust. It all went together very well. You can get the exhaust with or without mufflers. An "H" pipe or "X" system would be a bennifit to the exhaust system. It makes it harder to assemble. But it will add some power.
You'll might want to raise the front end of the car a little bit to help the headers clear road hazzards. They tend to hang low.

Click below and go to page #4, post #77 scroll down a tad for exhaust pictures with the above system. You can see a little bit on the low hanging drivers side header. Car has not yet been raised to help clearance.
http://www.forabodiesonly.com/mopar/showthread.php?t=10508&page=4&pp=20

A decent rumble also comes from tight centerlined cam and reasonable compression ratios.