Re: 1986 Nissan NS70A
You likely did nothing wrong.
Aside from a crack in a casting, the only way water makes its way into the gearcase is through the seals.
It is quite plausible that seals that old have hardened, so changing the impeller may have disturbed the driveshaft seal.
I would get new seals from a Tohatsu/Nissan dealer and replace them. While you're at it, check the drain and fill screws (and their washers) -- make sure they aren't mangled by a previous attempt to use a screwdriver that was too small. They're cheap. While mangled screw heads won't cause a leak, it's sure easier to get them in and out if they're in good shape.
