Update: I disassembled the distro to see if I could see anything. Never got brave enough to go beyond replacing points in past but went ahead and got it all the way down to where I could watch the counterweights working. Didn’t separate the shafts but I get how that works now! It was pretty grimy in there, but everything seemed to be working okay. I noticed what my shop manual calls the “Resilex” piece (it almost looks like gasket material) had a loose piece and it looks like it is kind of failing and starting to break up. I don’t see that listed anywhere as a replacement part- any ideas what I could use to replace that?
I went ahead and reassembled everything, static timed it at about 18 BTDC, hooked up the light and when it warmed up I set to 21deg at idle and I was seeing maybe 34deg at 3500rpm(?) (vac advance disconnected) does that sound right? It was hard to read the pulley as I revved it and the reading seemed kind of jumpy at speed, but as soon as let off it settle back to 21 nice and smooth, and no more stumbling.
I did a brief road test and it seems fine. Did just messing with the distro fix things?