Do you have headers or cast iron manifolds? There are fewer choices for systems with stock manifolds vs. headers.
Big block or small block? Some systems sound better on one vs. the other.
Is it a coupe or a vert? Systems for verts are a little more challenging to spec out, and what sounds good in a car with a steel roof is different than that for an open top car.
Do you want the system to look correct? A correct looking system for a 69 would have the tailpipes exiting at the tailpan vs. the angled dump down behind the rear wheels for 67-68. There are also varying degrees of "correct". Some systems have tailpipes out the back, but the tips look nothing like 69 tips. Other systems look like 69 tips, but are larger diameter. Still other systems have tips that are as close to an original 69 as possible.
The best OE style system is from Gardner. They have sound clips on their website of the different systems they offer. http://www.gardnerexhaust.com/soundclips.html
You can listen and see what an OE style system sounds like. Not ideal, since you can't hear them (soundclips) from the driver seat, but at least you can get an idea what the stock diameter, single transverse muffler style of exhaust sounds like. They also have clips of the chambered style exhaust.
Stainless Works makes some decent systems that are pretty close to stock, and cost less than Gardner. http://www.stainlessworks.net/cart/i...th=475_476_237
This is who I went with, and though I am not 100% happy with it, is was a lot less than Gardner. I should have spent the extra money for Gardner... I wanted a stock sounding system with relatively correct looking tailpipes, and the only gripe I have was with the gyrations I had to go through with SW to get a system that fit my vert OK (they told me it would fit, and it didn't).
Lots of guys eith headers run the Flowmaster 17139 system; it has really nice looking tailpipes, but I can't stand the resonance of this system. Some like it, I don't particularly in a vert.
Magnaflow systems like Frank offered are nice and fairly popular. I didn't want two mufflers personally; I like the layout of a stock style transverse muffler, but that's me.
Pypes makes a few systems for our cars, but I couldn't stand the crooked muffler and tailpipes of their transverse system. Just a design flaw that may have been addressed by now.