Jeremy,
I'm not aware of a specific 261 manual. I have two 1954 261 engines that were replacement shortblocks purchased from Chevrolet, one in 1958 and the other in 1964. Both of them came with the later main bearings with tangs and no shims. One of them also came with the elastomeric rear main seal. The original 1954 blocks had main bearings with dowels, shims, and a rear main rope seal.
I believe that pre-68 Dave recently posted an image showing the straight-sided and curved oil pans.
Hoyt
