I don't think that a main 2nd/3rd bearing swap is an issue. The bearings are stepped (my 270's numbers from the book: nominal bearing ID #2 2.7145 #3 2.7445) and the 3rd bearing is also a thrust bearing. So things will get tight REALLY quick.
I would simply plastigauge all bearings. If you have a bearing knock excessive clearance is the main culprit.. rod or main.
I am not a huge fan or trying to describe pings, slaps, taps or 'sharp' vs 'dull'.
