Last time we assembled my 340, when we installed the oil pump dive, it broke the oil pump. I watched my dad install it, it wasn't like he pounded on it, just lightly tapped it in. Ended up with low oil pressure, pulled pan and found broken pump. Could also be the oil pump pressure spring. By chance do you have ARP main studs? One of the stud and nut on the rear cap will not allow the pump to sit flush, and you have to use a stock bolt.
