For most factory rings you can get away with the short to medium rings with a 50mm scope even if youre using a heavy barrel. However, be aware that sometimes shorter rings will make you crinck your neck too much to get proper eye alignment. This will of course depend on your stock design and how you head and face is configured. If in doubt, get AT LEAST the medium.
Also even though youre using a one peice rail, often times when you ttighten the screws down when your action isnt perfect, the rail bends a bit. This can put stress on your scope. For a one peice base, you can tighten down the front and NOT the screws in the back. Using JB weld you can fill the gaps in the back. Be sure and use a release agent on the action and screws so as to not make the base a permanent fixture to the action. This way, when you torque the screws down the rail is perfect and you will get perfect scope alignment and prevent any damage to your scope. Personaly I prefer to use a 2 peice base and after mounting the bases I hone them together using a precision grinder/honer. This creates a perfect alignment. It also helps to lap the rings for an even better alignment. This also hones down any imperfections that can leave ring marks on your scope.
Hope that helps!