Poor idle and off-idle performance after distributor swap

Ima thinking you have several problems.

1) firstly; the arm on the Vcan fixes, that is; sets the position of, the advance plate relative to the signal generator. If your new Vcan arm fixes in a different position, the timing will be off. no big deal at idle, you just reposition the D and away you go.
But, When the Vcan starts to pull in timing, the rotor may get out of sync with the towers, and eventually it may spark to the wrong tower. As the rpm goes up, the centrifugal timing may exacerbate that. To correct this, you have to "phase" the rotor.
2) You didn't say, but there are CCW and CW driven distributors. I hope you knew that, and have the appropriate parts.
3) You have to figure out why, and fix, your jumpy timing .
Do this: Set your timing to about TDC* Crank up your idle rpm as much as you need to but get the idle set to as slow as it will run, in the 500 to 650 range, not a big deal
Aim you light and slowly increase the rpm about 200 to 500. Watch the strobe action.
If the timing jumps all around, or if sparks are dropped, or double sparks occur, then
Then the polarity of your magnetic pick-up is reversed, and it is generating erroneous signals. Your engine will never run right. Reverse the polarity, and retest.
4) once you get that/those fixed, reset your transfer slot exposure under the primary throttle blades, to about but not less than, square, and after this, LEAVE the speed screw alone. Set the idlespeed using idle-timing; Don't even hook up the timing light yet. Just push/pull on the Vcan until you find a sub-800 rpm in P/Neutral, that your engine likes. Set the mixture screws to about in the center of their range. Make sure the secondaries are closed up tight but not sticking. And then, with a fully warmed up engine running ~180*F, lower the rpm to in the range of 550 to 650 in gear, USING TIMING, NOT SPEED SCREW, as low towards 550 as it will idle smoothly and stably.. Do not even look at the timing, let the engine tell you what she wants, rather than forcing your opinion on her.Make sure the Vcan is hooked to the Sparkport. Now, take her for a test drive. Do not go WOT. You are just testing the idle to off-idle transition,just a gentle roll-on, looking for a hesitation.
If you do not find a hesitation, yur done this part. But
5) if you do get one; turn the speed screw in 1 turn, and if the in-gear idle is increased beyond 650, then retard the timing, until it drops back into the 550-650 zone..... and roadtest again.
If you still get a hesitation, twiddle the mixture screws for best lean idle, then add 1/2 turn rich, and roadtest.
If you still get a hesitation, add 1 turn more speed screw, reduce your in-gear idle speed back into the zone, by retarding the timing, and twiddle the mixture screws as before.... then roadtest.
It better be good by now.
But if it is not, gradually increase your idle speed in steps of about 50 rpm atta time, using TIMING, not speed screw, road testing after each change. Do not increase past 650 in gear, for no other reason than, it might clang,bang,or lurch from N/P to in gear..
7) After you get this all dialed in; NOW is the time to check the idle-timing, and to limit your Power-Timing, and to set your Rate-of-advance, specifically for your engine.
8)And finally; to get your cruise-timing up into the range of 42 to 56 degrees, to get rid of that lousy 11.5mpgs. Well, unless you have race-gears, lol.