I'm having similar problems with a sbc that I rebuilt so my advice may not be worth much.
I don't think you should be using a breather with a pcv valve. It should be a closed system. Try plugging the hole in the intake where the pcv goes in or plug the hole in the rocker cover where the breather is.
If that doesn't help I would pull the distributor and replace it with a HEI unit. If that makes it run smooth you can rebuild the old dist and put it back. It may be the original 63 distributor bushings or the gear are worn out. If it runs the same with HEI you'll at least know it's not the dist causing the problem.
There are some guys on here who know a lot more than I do about troubleshooting an engine so they may have more advice when they check in.