Pull all the spark plugs and prime the motor again, rotate it slowly, if the oil pressure comes up and drops off again it is the roller lifters Been there done that.The last motor we had that on we where able to block the oil off to the drivers isde and that cured it I use to small welsh plugs(look like small freeze plugs ) that I inserted into the drivers side oil galley behind the oil galley plug at the rear of the block It is a lot of work to either remove the motor and do it on a engine stand or remove the trans and do it in the car Try rotating the motor while priming it first before deciding what the problem is and the causes