Facing jig
It's quite rigid. In between the two tubing sections held together by the C clamps, I have a stack of feeler gauges, held in place by the dark grey C clamp that is my depth gauge. I added one more feeler gauge to set the depth of cut, set in the grinder, and c-clamped together. For each pass, I loosened the grinder, changed out the final depth-setting feeler gauge, and set it back in place.
The outside-edge-only paint removal pattern in this picture is from an earlier set of passes where I was using a smaller 4.5" grinding wheel that couldn't hit the whole flange. This one has much better coverage across the face.
Process
This is partway through my passes. It's a bit hard to see, but I've already been taking metal off at the bottom corners of the block even prior to this pass, and after this pass is done, I'm still not yet all the way through the paint at the top.
Forces that would tend to want to deflect the arm holding the grinder were quite minimal. I took about 5 thou a pass, and the edge of the disk was cutting far more than I was grinding with the face of the disk. I turned the crank from the front end, so as to not load the fixture with any of the torque needed to overcome the internal friction in the engine.
Results
Overall, looks good. Process went exactly as I expected. First passes grazed only at the lower corners of the block, while the disk spun freely and with no contact, at the top of the block. As I substituted thicker shims, I worked progressively further into the block at the bottom corners, and the disk kept contact with the block further and further towards the top of the arch on each successive pass. The final surface is smooth, very minimal surface finish irregularity. Its not mirror polished, but it's close. You can see a few places with slight grooving where I had to pause for a moment to click the wrench a bit further, but it's not even enough to catch a fingernail.
Now the big test -- to go borrow the dial indicator again and get a chance to check if it truly works now.
