50 MOA of elevation available after a zero or 50 MOA total?? What's your zero distance? I haven't run any numbers but I don't believe 50 MOA will make it from 100yd to 1600 no matter what, probably not even 65.
I'm going off the specs for my 50bmg, 338 Lapua should shoot as flat as the 50. zeroed at 200yards it takes me 38.75 moa of elevation for 1600yards. your best bet for a sloped rail would equal 1/2 of your usable moa range.
fmsniper-- You have 50 MOA total, not 50 from your 300m zero. You will have 20 to 25 MOA from your 300m zero, so a sloped rail is a must if you wnt to reach 1600... I would get at least a 20 MOA rail, possibly even a 25, if available. This will put you in the bottom revolution of your elevation turret, allowing you to use nearly all 50 MOA of internal scope adjustment.
Side note - Before getting anything over a 20 MOA tapered base, you might want to fit a standard base with the rings you plan on using to see how far off the bottom you end up. You "can" always shim it when you bed it if you can't get get it boresighted at 100, and I would check it first no matter what MOA base. Ended up 6 MOA short of the 100 yard zero on one of my Dad's rigs here the last time, had to re-bed with a shim under the front. Aluminum tape works well for a shim but, I don't recommend it but for just for a small point under the base the size of a match head or so with multiple layers just to hold it up while the bedding cures. I think a layer of epoxy between the two is much less compressive considering the glue on the tape.