While the Gardner systems are beautiful reproductions, they're not exactly performance-oriented.... pipe bends are rather restrictive, as are the 2" tailpipes. Adding a performance-enhancing crossover still won't bring the performance up to the level of a decent mandrel-bent system. Downside to the performance systems is they likely don't fit as well, especially with stock exhaust manifolds..... customization is likely required.
I used a 2-1/2" Magnaflow stainless system on my red '66. Trimming the tailpipe sections made it fit nicely at the rear of the car, but it hangs a bit low in the center of car. Also didn't like the loud mufflers, so I replaced them with Walkers.... sounds just right now!
Contact Pypes Exhaust. One of the guys there (Dave?) has a '66 GS they built a system for. They may have downpipes that will fit stock manifolds. Many options for pipes and mufflers to fit your performance/sound requirements.
Dan, does Waldron offer full 2-1/4" or 2-1/2" systems? That might be the best compromise for price vs performance