Detonation might have beat the bearings to the point the oil clearances are shot. Would cause knock and low pressure.
Sometimes "budget" rebuilds will mean the crank was simply polished, not turned to the next undersize. Have the crank measured for straightness, journal roundness/taper and diameter as well as checking the rod big ends to see if they need sized. Replace bearings all the way around to whatever size the crank ends up being.
If the rods need sized, replace the rod bolts w/ARP hardware before resizing for insurance.