Hi my name is Charles, I will assist you today. These questions require a rating. If you are satisfied with my answer please rate 3 or more smilies.
Since the case will be of you should change the gear oil. You will need 1Qt of that. Also some good marine grease for the splines of the drive shaft and shift shaft. With the case off be sure to check the tuner for being loose. I see that a lot.
This is how I replace a case.
With it in Neutral trim the motor up and remove the 5 nuts and the bolt from under the trim tab. If the speed-o is hooked up unhook it before removing the case. slide the case out of the drive shaft housing. Using a good strong vise hold the case upright and replace the pump. Make sure that you put a little Vaseline in the cup of the pump body. Install the impeller on the key on the drive shaft. Slide the body down while turning the drive shaft clockwise, press down on the housing until seated. Reinstall the nuts and bolt. Put grease on the splines of the drive shaft and shift shaft.
Reach your hand up in the exhaust to see if the tuner is still in-place. You can also look up the exhaust and see it the 4 nuts or bolts are still there.
This part will require a helper.
Lock a small pair of vise grips to the front of the shift shaft. Slide the drive shaft into the opening where it goes. As you come up make sure that the water tube goes into the guide on the housing. When you get to a point where the drive shaft stops have the helper bump over the starter, this should align the splines. As you get close to seating the case grab the vise grips and align the shift shaft. You may need to wiggle the grips some to get the splines to align.
Not too hard takes about 1 hr to do it all.
26 is the tuner