My mild(very by you guys standards lol)496 has never been on a chassis dyno so I can't back it up.I have 1 7/8 headers (C body) with 3" from the collector to the x-pipe and then 2.5 back from there.Flowmaster mandrel bent tail pipes from a B body with some extensions before the turn downs.My X is an old Dr. Gas x pipe that is 3" in / 2.5" out.In fact the whole system came from my old Charger 15 years ago.
I hardly ever drive it anymore but if it would pick up 20hp and tq I might would redo it.
If I had to guess I would say mine makes 525 at the flywheel and maybe 550 tq.