You can remove the seal and install the new on with the front cover still on the car, I have. I prefer not to do it that way though, because I like to see and feel all the way around the seal as its removed and installed as to not damage anything taking it out, or putting it in.
I would remove the cover, and do it on the work bench.
Getting it out is easy, and like said, don't scratch, or gouge the aluminum of the front cover.
When you install it, it goes in DRY, do not put any lube on it. The green color is the seal already installed, or a color/coating similar.
Press it in evenly, and smoothly. Avoid hitting/pressing hard as the front cover is made of very weak pop tin aluminum, and will crack, and fracture easy if you strike it too hard.
There is a GM J-tool that you use to install it with the cover on the engine, as it threads into the end of the crank, and you turn a big wing nut, and it pulls the seal on into the front cover easy as pie. I have always used one of my brothers super large sockets. Hes a diesel mechanic, so hes got some big ones.
Hope that helps.
~F~
