Hard to visualize it in my head to explain to you, was not able to find a picture online, but look below your alternator idler pulley. (the one to the left of your alternator that just spins, not attached to an accessory) Look down and there will be a pulley with a plastic cover on it (not always) It has a hydraulic piston on it. I think on the M42 it is just a standard bolt. Some motors like my M50 use torx, some use hex. Put a socket on it and turn it as to tighten it. This will release the pressure on the belt.
As for the clicking of the starter, it is a common problem. 95% of the time, the starter needs to be replaced. I did my own engine swap but I paid someone to do the starter.
Its something I would rather pay for, not worth the cursewords and blood to do yourself.