Just bought a 8 3/4 chunk for my 340. It is a 741 case. I have sprayed it with abunch of wd-40 and taken a small wire bruch to the ring and spiders. I will keep hitting with a bunch of wd-40 and let it drip. What else should I do? Any other things to clean it up before installing it. Do I need to grease everything once it is clean<? What gear oil is everyone using and how much?

Blow it off with compressed air and install it.

If the third-member is really dirty, you can spray it down with oven cleaner, then power-wash and blow dry. Be sure to squirt some gear lube into the carrier and pinion bearings before they start to rust.

How dirty is the thing? You could take it apart and clean it then put it back together if it is really bad. The 741 uses spacers not a crush sleeve so you should only need to have to set the side clearance / backlash.