93 lumina, low oil pressure and knocking...

ok so we have 93 lumina 3.1 with 80 some thousand miles on it... and it was given to us by one of my dads coworkers with a BLOWN head gasket... there was a mixture of oil, antifreeze, and muck all through the engine... we tore the engine down to the lower block.... cylinder walls were still in perfect shape so we flushed out everything we could with k-1 and rebuilt the top end... headgasket, lifters, pushrods...all that good stuff... well the car runs decently. and the exhaust is clean... but it knocks and has low oil pressure.... when you start the car up it sounds perfect... but as soon as the engine warms up i start hearing what i assume to be lifter knock.... when in gear the oil light blinks on every time it knocks.... which is every half second when idle....i put a half can of seafoam in the oil hoping mabe it would work out the problems but it hasnt sofar....but ive only ran it about an hour.... im wondering if it could be as simple as replacing the oil pump assembly..... has anyone had this issue or know anything about any of it?