The case letter is located on the back top right of the upper engine case. Where the engine holding bolt goes through. With that letter and the crankshaft marking you can determine the correct replacement bearings for the engine. With that said, as others have mentioned, I would mic out the crank and check for runout on the crank. You can have some abnormal wear that would cause the crank journal to be tapered from side to side, which would require you to have the crank ground. You can determine that by torquing the main bearings (without the crank in) and then using an mic measure the gap and compare that to the main journal, this will give you a really accurate clearance gap, then you can order the bearings that will give you the desired gap. You have to measure each one (I would record all the numbers) since you can have a variation from one cylinder to the next.