My guess is it's leaking between the aluminum drive shaft stuffing box and the fiberglass hull. If you dry out the bilge completely, put the boat in the water and look around this area you will see water very slowing seeping in. Like a pin head sized drip of water oozing up from the fiberglass mat.
My 89 was leaking in this area, it took a while to find and was confirmed by putting the boat on the trailer and filling the bilge with water and then looking under the boat for water coming out. I checked all of the 'usual' suspects - exhaust, water intake, rudder, etc. before I found the true culprit. Once I fixed this, it can sit for days in the lake without taking on ANY water. With the leak, the bilge would have several inches just sitting overnight.
Here are some links: