valve chatter 360 magnum

With a stock oil pump, the pressure is fine. I agree with your builder, put some 20w50 in there if it bothers you. Will probably clear up the chatter you are hearing at idle.

Comparing it to a slant oil pump is apples to oranges as the stock slant six oil pump is “high volume”. Also if Dutra worked his magic on the oil pump, it was higher than stock anyway.