I don't think the chambered is as loud as some others believe. It's no louder than any of the other muscle cars from the late sixties.
There is some questions about the performance
of this 30 year old technology. There are a quite a few crimps and curves and I believe the pipes are reduced to two inches in some areas. I prefer the two muffler set up with an H-pipe. The Flowmasters, two chamber "Delta Flows" mufflers with 2.25 pipes has a smooth tone and will outperform the chambered system by far. Your stock small block will love the two and a quarters which will be much quieter than 2.5" Flows. If you're running a big block or 450+ hp, you might go 2.5 inch pipes. Get the Delta Flow mufflers from Summit or Jegs for about $75 each, then take them to a muffler shop who knows what they're doing. They should charge $250 to $350 for the pipes and labor. The 69 tailpipe exit is diff from 67-68 so take some pictures to show the pipeman. Then I'd watch the work. They'll encourage you to leave the car, but I wouldn't. I'd watch their every move. Or, you could order a complete kit made by Flowmaster...pipes, mufflers, brackets, etc, but I don't believe they make a 2.25" kit. Lastly, for 69, order offset in, offset out muffles, which work better for routing the tailpipes between the quarter panels and leaf springs. Good luck with this.