For my '62 with the 327/SM420, I went with the Ram Horn manifolds and custom dual exhaust from Midas Muffler and used Magnaflow mufflers.
On my old '63 with the 283/327/400 engines at various times in conjunction with a TH400 tranny, I had a set of headers and a custom dual exhaust with glass packs.
Both sets had the mufflers outside the rails and under the cab and exited just in front of the rear wheel.
Both sets served me well. The problems I had were with the headers. I got tired of constantly having to replace exhaust gaskets and periodically replacing the headers.
I would advise sticking to the Ram Horn manifolds. They may not be as free flow as headers, but they will help develop torque and you will have less trouble with them.
