Pardon my ignorance, but what difference do glasspacks vs. mufflers make? I mean in terms of flow, backpressure, hp, mpg, sound etc? They're a lot cheaper...

 

When you dual up the exhaust you should get some power increase and better fuel mileage. The glasspack muffler wil be loud and has fairly low back pressure to help in fuel mileage. If you want a nice sound, a super turbo type of muffler works well - make sure it is a low back pressure muffler what ever you do. Any one of these types are far better than your restrictive stock muffler and make your truck sound like it has power.

 

So if I buy long glasspacks (e.g. 4 in. round body, 40 in. body length, 45 in. overall length, 2 1/2 in. inlet/outlet), I can get a quiet exhaust with the high flow of a racing muffler for half the price?
http://store.summitracing.com/part.asp?ssid=LWLCJM2ONDN42QYNW28NKK81GX&orderid=J H7HI5IJ2PKX1QQJFPJY3CFCUGTQ518SXJEN&part=2D25CD6A-3630-11D3-949D-080009FC5B5F