I guess it is a little too late to make this post now but here goes;
If your main caps are walking and showing signs on the cap mating surfces of metal transfer, I suggest to look at the crank closely for any color or bearing material on it. If there is or if the bearings show any shrinking find a shop with a Wet Mag Machine and get the crank Wet Magged. Even the highest dollar crnks cn get cracks when they get hot and the main caps move.
Are you torquing the main caps according to the Dart Specs? I suggest to go to the Reher Morrison WebPage and check their specs for their Super Series of engines and torue accordingly.
The example you mentioned of the main cap moving causing a bearing spin? I am wondering which came first. Usually it is a bearing over heated that causes the main caps to move.
Also I hope that you or whomever the machinist is checks the main cap to block register and see if it is still tight. You should be able to measure the distance between registers side to side and measure the main cap and the main caps should be measuring .005 wider. This gives a good tight .005 intrference fit. If you do not have a good tight interference fit, you may have found a part or most of your problem.
I do not suggest to use any oil restrictor in the front smaller than .125.
Ed