Hey Mart, my guess is that there was not enough lubrication and/or clearance on the rear thrust surface. When you press the clutch in, all the load goes to the rear main thrust - and it burned.
Was this the engine with the very-low oil pressure? You should never have less than 10 - 15 lbs at idle (I know I never have with any engine) - more like 20 - 25 even when really hot. If this is the case, then when you're idling the engine at a stop (with no oil pressure), then the rear thrust isn't getting oil, so the lack of lubrication could easily cause the bearing to heat up, burn and then be wiped out.
Another cause can be too tight of a thrust clearance - I like to have about .005 to .006 or so. If you tightened it up too much with a shim in the front of the rear shell, this might also be a culprit. Did you plastigauge the mains to see what clearance you had in the rear? I like to have .0025 to .00275 in the rear - especially if I'm going to be racing it as the rear main gets hot from the clutch, so the clearances will get tighter and this can cause lubrication issues. The rear main is the one that will cause the most issues.
A little "trick" I do is chamfer the edge of the rear main bearing a bit - to help get some oil to the thrust.
Here is an example of the 284 CI engine in my 32 CAB:
MBRear chamfer.jpg