The problem with the '54 full-pressure engines is that they're the shim-style main brngs. Perhaps improper shimming, or a streached bolt, incorrect inserts*, cracked cap, or oil starvation was the cause. All 4 of the 235 mains are pressure-fed from the main galley. Perhaps the oil hole in the upper insert didn't line up with the oil hole in block? Just one of many things to check. A shim-style 235 is easy to rebuild, but tricky to diagnose when a bearing fails.
Unfortunately, when an engine has a bearing failure, its best to completely teardown and rebuild. Careful examination of the bad parts will provide clues.
chip
* the 53-55 shim style engines used different main bearing inserts than the 55-62 235s. The 53-55s had dowel-pin inserts, and they shouldn't be able to spin at all. Inspect old bearings closely for correct application and undersize.
