Try Walker Quiet Flow Mufflers. Similar to the old Corvair turbo design. Nice little low sort of rumble at idle, not loud at WOT and no droning whatsoever. Mount them as far back as possible and yes, I agree, a cross over would help even more.
As for power, probably a little but you need to be realistic with this motor. These are not giant power gains. Small motor small gains and in some cases very small gains.
I have run Clifford tube headers on my 235 and 261 motors for over 40 years. One of my vehicles has a set of old Hush Thrush mufflers and smallish 2 inch exhaust. Runs pretty well. Uncork the headers, it runs even better but the design of the head and the lack of real ability to easily add compression has its limitations as to power enhancement.
