OK, thanks to this great forum, I decided to put the Arvinode on our family 65 K GT Vert. It just had a bad manifold to H-pipe gasket, but mission creep led to a new H-pipe from NPD and exhaust back from there. A couple notes regarding the installation:
1) NPD was not clear on selling me the correct system for a 65 GT so I bought straight from Waldron's
2) The H-pipe hung down way too low, so I had to have a local muffler shop put an extra 7 degrees of bend into the intermediate pipes between the H and the mufflers, making more room to pull the mufflers and therefore the H-pipe up closer to the floor
3) Waldron's agreed to put a 2" exit on the resonators to accommodate some non-standard diameter piping going into the GT tips
4) I welded the H-pipe, intermediate pipes, and mufflers into a single piece, then used 360 degree Torctite band clamps to attach the rest of the components back, should I want to disassemble more easily later on.
5) It has a great old school sound. Pretty mellow up to about 3K, then you either get a nice burble up to 1/2 throttle, or a full-on rap above 1/2 throttle. The rap is a bit much, but every once in awhile it's pretty entertaining.
5A) Mine might be a little louder than normal due to the louvers being gone from the tips, and the fully 2" exhaust through the tips.
I put some clip on Youtube so fellow members might benefit, may need to cut and paste into your browser.
I have a few install pics too if anyone is interested. Thanks to HiPoMustang forum members for all the great exhaust info!