This is something Daddy showed me when replacing a water pump.
Use the size bolts that secures the pump to the foot but longer (in this case the heads cut off). After putting the impeller to the shaft then slide the pump down to the impeller and rotate the shaft (in the correct direction)and it aligns itself.
I've been wrestling these pumps and impellers for years and worring if the impeller key fell out of place. NOMORE *;D