mufflers or no mufflers??

Those look like the early 273-318 manifolds. I'm far from an expert but I think those are just to small to flow well at anything over 5000 on a 360 size motor.
I had a 383 rr motor in a cuda with C-body manifolds and it was a turd above 4500 but when i put headers on it would pull hard right past 6000. dosent tti make headers for the early "A"s now. I know there pricy but u spent half of a set on a glorified tune up and got nowhere. Just 1 opinion.