I've got a CZ 550 Magnum which I'm going to top with a Leupold VX-III 1,5-5x20. As you might be aware of, the stock of this gun is falling, which means it's almost to low already, even for the iron sights. Then, what mount do you recommend? It has to be as low as possible, and since the gun is a 416 Rigby, the mount has to be rigid as well.

You probably don't have a lot of options. There are the CZ mounts, and those from Talley's and Warne, if you want to stay in most rugged group. None are low. I've got Talleys on my CZ but then I have a MacMillan straight combed stock. They're OK for me with this stock.