Caravan 3.0 Oil Pump question

1997, 140kMiles

When doing a valve job recently I also replaced the front crankshaft oil
seal. The parts store sold the seal with the gaskets for the oil pump as a
set. No way to get the seal alone. Alldatadiy.com showed the exploded view
of the oil pump housing and looked simple to remove the housing and then
replace the seal. Not so!



With the housing was loose, it would slide forward a quarter inch but that's
all. I ended up using RTV to seal the housing and bolt it back in place. I
removed the seal careful not to scratch the shaft and got the new seal in.
(the old one was hard, as expected). no leaking thus far (only a week hence)



Anyone know why this would not cooperate and slide off per the illustration?