gas tank so you could easily just drain that. Yes the water pump and cam seals arent necessary unless you know the motor is good. The point of changing all that is you dont want to have to take all that stuff apart again a month later when you could have already done it. I would turn the motor clockwise by hand a few times with a socket on the underdrive pulley and recheck the belt on the tensioner side. That should make it a little easier to remove and replace the tensioner. And of course make sure the battery is disconnected to prevent accidental starting.