Changing power steering pump

[Mack B]

How difficult is it suppose to be to get the pump "seated" into the reservoir? I watched a few videos and they where able to push them in by hand. I of course had no such luck. I tried tapping around the outer flange with a hammer with no luck, finally a dead blow to the pump itself got it close, but the o ring is visible along the top edge and I can pry the two apart by hand.

This is a pump for an 87 R10 on my 63 C10. Anyone have any experience with this?

First picture is with the two sitting together, second the pump is beaten into place. Third picture shows the top edge not seated deep enough.