Block out of square

i would bolt the flywheel on and attach the base of the dial indicator to it and measure the back bolting surface again. After that I would bolt on the bell and measure the surface the tranny mounts. The other thing you could do is to use a straight edge and measure between it and the flywheel in two places. It’s hard to believe that a production block could be off that much since both surfaces are probably machined on the same piece of equipment.