I was looking to replace the impeller, followed the manual instructions for removing the lower unit. Removed two bolts per side, a nut, and disconnected the lower shift rod screw to no avail. It's stuck, any suggestions? There looked to be some corrosion around the stud toward the front of the foot, so I put some penetrating oil to work on it. Please help! PS Forgot to add, this is a 1977 35hp Evinrude.