If you can roll one of the bearing shells out of either the block or the cap, the undersize should be stamped on the back of the bearing. That's almost as good as measuring the diameter of the shaft. You definitely need to determine what size shaft you're dealing with before buying replacement bearings. If you're doing a full overhaul, the crank has to come out for cleaning and polishing anyway.
If you have confidence in your guy's ability, that's good, but I simply cannot imagine anyone telling a customer that he can use different size bearings than the crankshaft diameter. Maybe I misunderstood that part of your description of the situation.
Jerry
