Thanks for everyone's comments.
To keep the exterior completely original and not have sealing washers under the castellated nuts, I like Lawrie's solution of using an O-ring around the 7/16 inch stud that fits into a 1/2 inch hole at main bearings 1 and 3.
The wall clearance between the stud and wall for main bearings 1 and 3 is 1/32 inch.
During the design, I tried to find 1/32 inch cross-section O-rings that would work, but they are not available in the diameters needed.
Lawrie's post made me look at metric O-rings, and they are available in diameters that I think will work.
A 1 mm cross-section O-ring is .0397 inches.
The 1/2 inch hole is 12.7 mm.
My thoughts are that a 1 mm (.0397 inch) cross-section x 12.5 mm (.4921 inch) OD O-ring will work. The ID of this O-ring is 10.5 mm (.4134 inches) and will stretch when installed on the 11.1125 mm (.4375 inch) stud. I'm thinking is that the stretch on the ID will increase the OD to be greater than 12.700 mm (.500 inches).
McMaster Carr O-ring 9262K156 or 1295N129 are both 1 mm cross-section x 12.5 mm OD and oil resistant.
Other O-rings that are 1 mm cross-section x 13 mm OD will work but they may be slightly harder to install because the 1 mm cross-section will need additional deflection.
McMaster part numbers for 1 mm cross-section x 13 mm OD O-rings are 9262K163, 1174N102, 1247N102, 1171n102, 9263K549, 1295N131, 1185N12, and 5233T19.
This information will be added to the next revision of the "Builders Guide" found at
www.modelaengine.com.