I'm still thinking about the order of assembly.
When I took it apart, I pressed the shaft out through the seal, then removed the seal from the pump housing. When I tried to go back together, I did the reverse, installed the new seal into the pump housing and then tried to press the shaft into both the housing and the seal....didn't work, no way to support the seal without damaging it.
Maybe Ron's idea, freezing the shaft, along with soaking the seal assembly in hot water will work.
I would have first wondered if the fitment on the shaft was to keep the seem between the seal/shaft water-tight,
I don't think the seal has to be watertight to the shaft, but it does have to be tight enough for the ID of the seal to turn with the shaft, so it does need to be a press fit.
I need to do some very careful measuring, time to put away the calipers and get out the mics and ball gauges!