Q..Have you tried to run an impco 425 ? It may work better on a 400 cu in engine.I also went two steps heavier on the advance springs that on paper have everything in (with the black bush) at around 2500 rpm. However this does move the start point of the centrifugal well after idle.
The issue you have is you still get a slow curve rate with the bush.. go to using stops as I described above, and light springs.. basically you go as light as possible but not light enough for the springs to bounce at idle or fast(choked) idle. If you do, the advance increases, the rpms increase to a point the idle mixture circuits cant supply the required fuel, the rpms drop and repeats. you will find that usually cent all in at 2500 rpms will bounce the weights.. 2600 pushing it, 2700 good.
So timing aside I reckon I still have something a little weird going on (some sort of restriction?).
If was a 425 I would say remove the centre piston and check it and the bore for the most tiny of machining burrs.. a very light rub with around 1600or 1800 wet dry . Many yrs since messed with a 300...I would guess a similar situation inside that.
In regard your timing issues, the msd is a bit tougher to get to work with a low idle fast curve but there are ways around. An adjustable vacuum pot helps. I have used the heaviest and lightest spring in combination with a slightly higher initial. Heavy spring held off the curve a bit longer then went all in quickly due the light spring at about 2800. Vacuum filled in the middle. It worked for me but I was at much higher compression. Does your engine run well with a higher or lower initial?
Otherwise to get the low initial fast curve, an hei style distributer is much easier to tailor.