If your manual is for a 1953, I don't think that it is giving you the correct instructions for a 261, maybe a 216 or older style 235. I don't think that the 261 would use shims in the main bearings.

1951 GMC 1 Ton Flatbed -- It is finally on the road and what a great time I have driving it!
1951 1 Ton Completed

My Chevy Master 4 Door is on the Road!