My 340 in my 66 developed issues :-(

I would get matched manifolds, clean them up and install 2.5" downpipes to match the rest of the exhaust. That itself should be an improvement and I think you will be happy with it since you want it mostly stock looking. I've got Hooker Headers on my Duster because I could not afford TTI and I need headers for my combo. But they are hard to get a good seal and they hang way low to the point I'm thinking of going back to manifolds.