Diagnosing cylinder misfire

I have a misfire off idle as well. In my case I believe my OEM CAP distributer (273 2bbl, OEM points dist) is advancing too much if I have the advance set to greater than 0 deg. (OEM spec it 5 ATDC) the today timming at 2000 rpm is around 50. ( I had a timming tape on it for a few hundred revolutions)

Also the dwell gets really small when the vacuume advance is fully advanced.

I would test again with dist at 0 and no vacuume advance and check the dwell through idle to 2000 rpm. If the misfire goes away that might give you a direction.