I just replaced the impeller and gaskets on my mid-70's merc 500 water pump. I can get the lower unit back on the motor but the shift splines don't seem to line up right, when together shifting into nuetral or forward and reverse the prop turns in the same direction no matter which gear it's in. I had it in forward when I took it apart and did not change it until back together, what am I doing wrong? Thanks, Tom