I just went out to my backyard range and fired six rounds. .457" ball over 41 grains (2.8cc Lee dipper) of 3F Goex, Crisco over the ball, Remington #10 caps. It shot fine. About 4-5 inches high at 10 yards, which is quite workable. The loading lever did not drop.
BTW McMaster-Carr has round shims (basically washers with precise thickness) in 3/8" diameter and various thicknesses. Not expensive, either. Thicknesses range from .001" to way thicker than we would need for this purpose, in mostly .001" graduations. I might buy some. It would probably be better in the long run to have a single shim of the needed thickness instead of stacked shims. Link:
https://www.mcmaster.com/products/shims/shims-2~/ring-shims-7/