Today was D-day for splitting the block and taking a look at what was causing all that racket. I pulled the oil cooler & pan off the block first, and could see some flakes in the bottom of the pan, so I knew it was definitely a bearing.
After pulling the various plugs off the block I hefted it onto the bench and removed the flywheel, then split the case and this is what greeted me:
Needless to say, I’d found the problem.
Next stop will be the machine shop to find out what will need to be done to get the crank into shape again, and have the block hot-tanked.
More pics are here.