It doesn't hurt to check the bearings for condition, but it will be just about impossible to measure the diameter of the shaft with it in the block. You won't be able to get a micrometer far enough over the main journal to get an accurate measurement. Do you know if you're dealing with a standard diameter shaft, or one that's been reground? The best guesstimate of main bearing size will be by using Plastigauge to measure the oil clearance. CAREFULLY checking the main journal with a dial or digital caliper will give you an idea if the shaft is standard or ground undersize, and most replacement bearings will have STD. or an undersize stamped on the back of the bearing shell. OEM bearings, not so much.
Jerry
