I develop my loads at 100 then tweek them at 300. I settle when my vertical stringing is inside .5" at 300. I've never found a load that wouldn't shoot good at 1000 when doing this.
Why not shoot 105 amax with maybe 1-8 twist? That would be my choice. If not, probably a 1-9 with 87 vmax. I do not own a 243 ackley, just guessing. Others that actually have used a 243 ackley would know better.