Couple of tips. First, I find I get less frustrated working on a G4 iMac by holding it between my knees upside down.
Second, getting the bottom back on is a bear, because of the multitude of cables, some of which are barely long enough and require dexterity and vast quantities of patience while reseating the bottom.
Third, there is a spot where you need to scrape off the old thermal paste (use something non-scratching, like a spudger) and replace it with fresh. So do some googling to find more thorough instructions.