960M main bearings are 56-63 and do not use shims. Not one, and not for any purpose. 56 got precision bearings, a neoprene rear main seal, and the 848 head. These changes raised the compression, horsepower, eliminated the shimmed bearings, and an improved rear main seal.
If your crank is locking up when you torque the No.3 main thrust cap, you have a bigger issue. Bent crank, mismachined crank, journal out of wack, main bearing bores way out of wack (not something these motors are known for but does happen), something.
Might be time to have the crank inspected, and if good, machine the crank to specification.
