The shims were used in the earlier engines with dowel pins on the back sides of the main bearings. If yours has the anti-rotate tabs on the bearing shells at the parting line between the block and the main caps- - -no shims.
Here are the part numbers for the later model (1956-62) 261 engine main bearings:
TRW: MS1721P
Federal-Mogul: 960M
Mt Federal-Mogul shop master bearing catalog lists bearing sizes as STD, .001, .010, .020, .030, .040, .050, and .060" undersizes. Some of those are probably obsolete now, as the catalog I'm quoting was printed in 1991.
This catalog also lists main bearing bore sizes, if you're interested. Bore size has a direct relationship to whether or not the bearing is round and has the right oil clearance when the bearing is torqued into place. We never relied on Plastigauge on the race engines- - - -all the rod and main bearings were torqued up and the ID of the bearing was measured with a dial bore gauge, then compared to the crankshaft OD. We would try for an oil clearance tolerance of .0002" or less, even if several individual bearing shells had to be measured. It was not uncommon to go through 5 or 6 sets of bearings to achieve that tolerance, due to manufacturing variations.
Jerry
