So is this is a first start of an overhauled engine?
If it is and you're using the stock oil filter mount. You know the threaded nipple and round plate with the holes in it. Anyway, remove filter, nipple, and round plate. there's a threaded hole under this plate that's supposed to have a pipe plug blocking it. If it wasn't replaced after cleaning block the oil pressure will be just as you described. The good news is you probably didn't damage anything. But stop driving it till you fix it!