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I file a nick at the front of each main bearing cap to ensure that I re-install it correctly oriented. The rounded raised ridge is a perfect place to do this. Painting a dot or slash can result in it being wiped off by accident because unless you get the cap squeaky clean, an oil residue will remain, which makes for a poor base for retaining paint. You do NOT want to swap the caps end for end! That will throw off adjustment. This won't be a problem with the rear main bearing cap because it is unique and can only be put back in one orientation. You will need a long 3/8" breaker bar for leverage and VERY shallow 3/4" socket to break those two nuts loose. As long as you have the oil pan down, why not also adjust the rear main cap? I'll bet you don't need to remove any shims from the front main bearing. I think the most I have ever removed from that cap was one thin shim from each side. Too bad the other two main bearings can't be as long-lasting and trouble-free as the front one!
Instead of instantly pulling the engine to adjust the main bearings - although I agree that it is much easier to do it that way - I think I would first want to know if the center main cap has broken Babbitt in it. If it does, that's what is causing the loud heavy knocking noise. If the Babbitt is broken, the engine will have to come out for sure to re-do the main bearings. If the cap Babbitt is still good, it's likely the Babbitt in the block is also o.k. Not always, but usually it's the cap Babbitt that goes bad first. Check the drained oil through a strainer for pieces of Babbitt, a sure sign that one or more bearings have gone bad. Also remove the dipper tray and check for Babbitt chunks at the bottom of the pan.
On the other hand, if you cannot remove any shims or only one thin one from each side from the center cap without locking up the engine, then the Babbitt up in the block is probably cracked. The kind of noise you describe cannot be eliminated by removing only one or two thin shims from the center bearing. It sounds more serious than that. Because the noise apparently started all of a sudden, I'm betting the center bearing Babbitt is history. 'Sorry for jolt, but welcome to the world of Model A Fords. The center main bearing is the Achilles heel in the Model A engine that also affects the rear main bearing and oil leakage. Cross your fingers and pray that the cap Babbitt is not broken and that you can remove a couple shims before the engine locks up. That means you just might have fixed the problem. Otherwise, you'll have to decide whether to have the mains re-poured or convert to modern insert bearings. Those readers who are familiar with my postings on two Model A websites since the late 1990's know where I stand on THAT issue! Need I INSERT my preference here???
Marshall