Bill, Gardner uses the same system for 65-66 GS.
Here's a pic of the original exhaust on my red 66..... amazing that it lasted so long, car spent it's first 3 decades in California.
Joe, the Magnaflow has straight thru mufflers, I believe 14" long.
For me, it was too loud at cruise....hard to hear music or have a normal conversation in the car. I did call Magnaflow about the higher-than expected noise level, but they wouldn't give me any trade-in credit for used mufflers, so I bought the 20" Walker super turbos. It did require cutting/welding some sections out of the mid pipes to accommodate the longer length.