Tunnaman,
Try this link (it should be a youtube video "Mercruiser Impeller Replacement" posted by barrysmarine). It's the best one I found.
https://www.youtube.com/watch?v=KfatYFUVy24[/url]
I found making the jig that Barry suggested made the job a LOT easier - it not only held the lower unit conveniently, it gave me a small flat work surface to keep the tools handy and to put parts on as i took the old pump apart. I couldn't figure out how to insert a picture of it - if you're interested in making one, PM me and I'll email you photos and a sketch of how I made it.
Add Spareparts' instruction about starting and keeping the system in forward gear and when remounting the lower unit, turning the prop counterclockwise to keep it in froward gear and to turn the spline of the drive shaft until it aligns and slides up into the upper unit. To be sure the lower unit is in forward gear when you're ready to mount the lower unit back on the boat, turn the prop counterclockwise slowly and grab the little splined shaft sticking up at the front of the lower unit and turn it clockwise with some pliers (be a little gentle - you don't want to screw up the splines) until you see the driveshaft turning.
Remember to get the two nuts on the sides threaded on before you get the lower unit all the way up tight to the upper unit (there's not enough clearance to get them on with it up all the way).
If I can be of any other help, let me know.