A 20 MOA rail should do the trick for you. If you have 25 moa of adjustment left with your 100 yard zero, that would indicate your scope is set at 20 moa from the bottom at 100 yards. A 20 moa rail would put you at the very bottom of the adjustment at 100 leaving the full 45 moa available for you to dial up..... that will get you well past 1000.
At worst, you could end up not quite being able to get the scope to go all the way to zero at 100. That is really not a problem even if you have to do a 150 or 200 yard zero. That will put you 2" or less high at 100. Unless you are trying to make precision shots on very small critters at less than your zero range that should not be a problem
You might check out the 300 grain Bergers as well, the higher BC should get you to 1000 with no changes to your setup. I am shooting the Bergers out of one of my rifles at 2875. With a 100 yard zero my dial up for 1000 yards is 22 3/4 moa. External ballistics does not change from one rifle to another, so if you run the Berger at 2900 you should be able to get on target at 1000 with around 22 1/4 to 22 1/2 moa. Your current set up is giving you 25 moa of adjustment so the Bergers should get you there........ but not much further.