Your next challenge will be getting to your exhaust shutters/flappers. You'll have to work the exhaust elbows off of the risers and that is no easy task for a boat that old. Be patient and work slowly to work the rubber boots off the elbows (they will be stuck with age). Be careful, too, because if/when you are trying to use tools to massage the rubber away from the metal, injuries can and do occur. I have been following your posts with great interest and hoping that you can get this resolved without putting alot more money into it, From the sounds of some of your past posts, you have endured some severe overheating. I imagine your shutters are melted/gone, and maybe in the bottom of the Y pipe (sometimes visible/retrievable through the exhaust boot when you take off the outdrive).
Next, because of the recent oveheat (and I think you said you have replaced the impeller because of that), you may have seriously damaged the waterbox cover that the water pump pumps into (this cover is plastic and is mounted to the underside of the upper half of your outdrive). If this cover is distorted because of melting, you will not get the proper cooling flow to the engine (on muffs, in water, either way).
Let us know how your waterbox cover and your exhaust shutters look. Please post photos (I recommend using photobucket website and then copying/pasting the IMG link to your posts.
Best of luck.