Jon,
Based on all of our measuring, comparing, and component swaps. I believe the only variable left is in the internals of the wheel cylinders. The piston, cup, or spring (or all) in the wheel cylinder must not be as thick/long as it (they) should be, causing you to have to crank the adjusters so far out. See
Fig.50-Wheel Cylinder Parts Layout from the 1948-51 Chevy Truck Shop Manual. Since the geometry all checks out, my recommendation would be find another source for the rear wheel cylinders.
Keep us posted.......
