And here is one other thing if you get to the end of the list and you haven't sorted it.
Even if your 261 was rebuilt and got a new camshaft and cam gear, remove the timing gear cover (not fun if you didn't change things so it can be removed without removing the oil pan) and gently tap your camshaft toward the back of the engine. If you are careful you might also be able to do this by removing the distributor and trying to move the camshaft using the distributor driving gear.
Then using a flat bladed screwdriver gently try to move the camshaft forward toward the radiator. There should be .001" to about .004" of clearance between the cam and the retaining plate...or thrust plate, call it what you wish.
If you can visibly see the camshaft move, that should not occur and it might be your problem or it might be adding to it. Your cam lobes are made so there is an angle to the rear of the lobe. That makes the camshaft push back toward the rear of the engine...but not always as efficiently at low speed idle it seems.
Twice over the last 60 years I've seen it happen where there is a lot more clearance between the cam and the retaining plate and both times it has made idle wonky. As the cam moves in/out it can change ignition timing and also to some extent valve timing.
I don't believe it takes as much as you might think. About 30 years ago I owned a 235 with a camshaft that had over .020" clearance between the plate and camshaft and it was to the point where idle had to be set up to nearly 1000 rpm to make it run. At higher speed the cam would move toward the rear but at 500 rpm it would bounce back and forth altering the timing constantly.
Good luck.
