On my outfit - when I got my 312 'Bird, the exhaust heat valve was stuck - could barely be moved - with force. And the little car basically had a single exhaust, with an awful heat load in the intake manifold.

I got good advice - for a desert car - to simply remove it and put in a spacer - which immediately caused the RH pipe to work. But, as you say - it seemed down on RH flow just idling in the drive. When I got my wife to jazz the throttle (she's had plenty of practice) while I checked - a whole load of crud came blasting out. Carbon, rust, spider webs, you name it. It actually needs a new muffler on that side I'm afraid - but it will have to wait on a total exhaust re-plumb I have planned.
I believe the "lack of use" situation causes the stock RH muffler to load up and the spacer is the best all around solution.
Steve Metzger Tucson, Arizona