OK, is the gearcase still installed on the engine or did you pull the gearcase down leaving the driveshaft up in the powerhead? The gearcase will probably pull down, forcing the impeller key to jam through the aluminum impeller housing….Not a big deal. Try not to force anything too much, or pry with sharp objects that will deform the mating surfaces between the exhaust housing and the gearcase… Unfortunately, this is not all that uncommon for these engines, they sit low in the water so the splines tend to get rusted together.