I use RL26 with 80gr ELD-M in my 224 Terminator, what my gunsmith calls his version of a 22-243 Improved, and it is sending those 80's down range at 3650. RL26 is one of those powders that produces very good velocities but as you already know, is very hard to find so unless you already have some you're chances of finding any are slim.
Another powder that will work well for you and much easier to find would be IMR 7828ssc. It probably won't give you quite the same velocities as RL26 but should be real close.