Oilpressure suddenly going low in new 414 smallblock

Help! Have a newly built 060 4" 360 stroker with Eddy heads, mech 0,55" cam, strip dominator, 850 Demon in a 65 Valiant. Has separate oilcooler / remote oilfilter. Had an issue with cooling water (pure water) leaking into oil, fixed OK. Changed filter, oil. Then after a few miles of streetdriving (some of it heavy!!) it suddenly lost oilpressure. Tried to pump oilpressure with a drill, and gaining slowly, up to some 50 psi at full speed on drill, but weak, and almost zero on motor when running. Max 20 psi at some 3000 rpm. No metallic in oil, checked old filter OK. Dare not run it longer. Could it be the relief valve in the pump stuck? (from the previously creamy oil). Anybody had similar experience?
Preparing to pull the pan anyway.
Thanks, anybody.....

The only option I see is to pull the pan and look.