I got the GT3 muffler together with the B&B 964 headers with heat installed on my 71.
I modified the rear section of the engine tin to make more space for the muffler. I used the SC engine console and cross bar. Made a custom support for the muffler. For the heat cross over pipe I use one from a 3.2 and modified it a bid. I also had to modify the bottom section of the rear bumper. I made a heat shield above the right side of the header and wrapped the oil line with heat screen insulation.
Here are some pictures of the system.
Here is what it sounds like
Cheers
Engelbert