Block out of square

I had forgotten about this statement in your first post:

"From there, my observation is that the bell itself has a scalloped shape compared to the block. Thus, there's only actual contact in the area around the mounting bolts. Between the bolts, the bell doesn't touch the flange at all. So my next thought it to get a set of precision shim washers, in a range of thicknesses, such that as a set, they make the amount of ramp I need to bring the mounting surface back to true. At that point, I believe my dial indication comes out at or near 0, the angular misalign is corrected, and I should be good to proceed."

If the bell mounting surface is scalloped like you state and only makes contact around the bolt holes then washer/shims should work?? Just to make sure though I would place the bell on it's face (rear face side down on a flat surface) and use a good scale to see if the front and rear surfaces are parallel. I can measure within .015 with a good machinist scale. If the surfaces are parallel then shims should do the job!! You then might need to use silicone (or something) to fill the gaps so that you don't get any rear main oil splatter from coming through the gaps. One reason the flanges are dead flat is for structural strength but also to create a seal. Treblig