I ground it down to create a central protrusion on it to easily line it up when driving.
You want the driver to come close to the edge of the brass seal so when tapping it in, it wont deform-warp the brass housing. Perhaps use the backing plate to keep it going too deep. If you go to far with it, you drive it all the way out and start over. If it is too deep it will hit the black carbon seal with too much force when you press in the shaft-bearing. You also want to drive it so it is not cocked or the seal faces wont seat properly to each other.
I also use some sealer. Either permatex #2, or teflon pipe paste on the brass seal. I also put a lot of lithium grease in the bearing pump support and outer bearing and inner bearing lip seals and I also moisten the ceramic-carbon seal with water or antifreeze
There is very little movement of the black carbon face inside the brass housing, so you need to get it lined up close to flush with the inner surface of the pump bore.
ALSO, you should press the shaft in and out instead of hitting it. If you hit it you can wreck the impellor drive end, SS metal is soft, then you will have to file it back to shape OR you can internally damage the double roller bearing and it will be noisier.
It takes just a little pressure to slide in and out. I rigged up something using a jack, some wood blocks and a boat trailer weight to press the shaft bearing in and out. A little tricky, perhaps risky!, Or you can use a large gear
Or nicer, a hydraulic press.
When the shaft with bearing is seated all the way, the white ceramic seal slightly depresses the black carbon face and the snap ring fits easily into the bore.