I bought an Olds 350 once that was "rebuilt," and it had low oil pressure. The bearings were new, as are yours, but upon further examination I found a new rod bearing on a wiped rod journal, with ohhhhhhh, what was it............ .030 clearance?! The oil must have been pouring through there........and it was just starting to knock........might not be what's wrong with yours, but I have learned to be very leary with previously "rebuilt" engines. Just my