I have that motor and have done many impeller changes. First off, replace the impeller, wearplate and the 2 gaskets that sandwich it, as a minimum. Check the upper pump SS cup for scratches. Replace if you find them. Check the pump base for melting, if so, replace it. If you are getting water into gearcase, shift seal is prime suspect, followed by pump base seals.
Clean driveshaft if corroded so impeller comes off easily. Use some soap on new impeller when seating it in pump cover. Don't forget to install drive key. Make sure base and cover are fully down. Cover is plastic, so snug nuts down, but not tight enough to crack cover. No more than one hand on middle of socket wrench, so you don't break pump. Install heavy rubber slinger on driveshaft and push down to contact water pump cover. Check for rubber gasket inside pump outlet. Put water tube guide tube into water pump outlet.
Marine grease on driveshaft and shiftshaft splines.
Make sure control and gearcase are in neutral. With motor tilted up, shove driveshaft into midsection. Insert water tube into plastic guide tube. Push gearcase up tight. Turn flywheel until driveshaft splines mate with crankshaft. Observe shift shaft thru front of midsection. Needle nose plier can align upper and lower shift shafts. Wiggle gearcase in place. As you do that don't forget front upper nuts, which need to go on pretty early. Use finger pressure to keep them snug. Do not tighten nuts with wrench to pull gearcase into position. So, wiggle gearcase, tighten nuts with fingers, wiggle, tighten...
You know about the bolt hidden under the zinc anode?
