At the very least, consider an ARMS 36 EX to extend the rail out...that's a long scope, and you'll need to move it forward some so your cheeck isn't all the way back on the buttstock. Plus, it'll give you a little more height, which will help with that 50mm objective...you can probably use med or high rings instead of ultra high.
If you can afford it, the LaRue mount IS very nice, and would work great here. Depending on the length of that scope, you might even need the EER instead of the SPR.
Just for grins, I stuck a Weaver Grand Slam 6x20x40 in my EER...it was *perfect* in terms of eye relief/cheek placement. At 15" +, it's a pretty long scope, so the EER helped move it forward...more so than the SPR would have done. Same deal with my Buckmaster...it's as long as the Weaver, but there's more tube behind the adjustments...and it worked very well with the EER too.
The eye relief on both those scopes isn't especially long, it's just that the scope themselves are, so you have to consider that when choosing a mount.