1993 F150 300 oil pressure problem

I have replaced the oil pump and the mains and rod bearings, which after doing so initially I had good oil pressure, but out test driving it and having only MAYBE 50 miles on it the oil pressure drops to zero and the clattering starts again. Does anyone have any ideas or had this problem and actually resolved it? Any help is appreciated.

Did you change the oil pump screen or clean it? Sometimes they can get clogged. Also, need to be very careful putting the oil pan back on, or could not the screen loose. Also, did you change the oil pressure switch? Sometimes they go bad and still don't leak. Hope this helps!