The reference to the Pontiac Main Bearings is the mentioning of the components that hold the crankshaft in place in the engine block. There are three to five of these main bearings in the engine on your Pontiac. The exact number is determined by the size and displacement of your engine.
The replacement of the main bearings in a Pontiac engine is not a very common occurrence. It is more common to be replacing the connecting rod bearings. If the reason for the replacement of the main bearings is because of a deep thud sound emanating from the engine block, it is best to investigate the connecting rod bearings before replacing the mains.
The measuring of the crankshaft and the bearing cap is required so the correct clearance for the new bearings can be obtained. This measurement is done with a micrometer and takes a reading to the thousandths of an inch. It is this level of precision that is required for this engine component to be correct when it is being replaced. If the new Pontiac main bearings are too loss, the crankshaft will have slop in it. If the bearings do not have sufficient clearance, then the oil that should be passing through them will be insufficient to lubricate them and the bearings will overheat and seize up.
The replacement of the main bearings on your Pontiac does require the person to be knowledgeable in this procedure and have the ability to know when the components are not correct for the application they are being used in. This will make it possible for the engine to operate correctly once it is reassembled. This is one set of components where guessing should never be done.