there is the possibility that your oil pump pickup is either coming off or developing a leak - or getting plugged up even. possibly your spin on adapter has a malfunctioning bypass valve & your filter is clogged? the only other thing i'd think would be a bearing going bad (rod bearing would not only explain the lower end knock, but also the oil pressure loss as well). best to be safe than sorry - yank the motor, pull the oil pan, check your bearing clearances. i know it's a pain in the butt, but - better to buy a new pan gasket & spend a few hours pulling the motor than to throw a rod & have to buy a new motor altogether. get yourself a plastigauge & check your clearances. there's really not a lot to go wrong in your oil pump, but - i trust you DID replace it when you did the bottom end, right? (i mean, come on, you're going to do all that & not put in a new oil pump? that would be like buying a new car but putting in your old oil filter or spark plugs only worse! the oil pump is the life pump of your engine)