Got some news, though we still haven't figure it out. We decided to take the head off and take it to a machine shop to see if there was possibly something wrong with it, like a bad valve job or loose valve seat or something like that. Well, it came back with a clean bill of health. Looks like it's something in the shortblock somewhere, though it sure seems like a valvetrain issue.
I do have a question. What path does the oil take in a B motor? Just a guess, but is it oil filter -> oil pump -> mains -> cam bearings -> lifters -> rocker shafts? The reason I'm asking is I'm wondering if it's possible we're losing oil pressure to the lifters for # 6 and 8. I seem to remember when we swapped the cam that the cam bearings looked like they've seen better days. Is it possible oil pressure is bleeding off at the cam bearing, making it impossible for the lifter to pump up enough to hold up to the faster lift and spring pressures with this combo? The engine shows good oil pressure on the gauge (60+ cold, 20 @ hot idle), but maybe that's not the whole story.