OK, well for that, you will want a broad torque curve, not emphasis on max HP. I've rallied for a long time and it is the same thing..... you'll never know what RPM's points you will need to be out of each turn as you change tracks and courses (or stages on a rally), and so a broad torque curve does you the best.
As high a CR as you can get will help extend that torque band down as low as you can get. If you want to keep those pistons, then I'd mill the present heads as much as you dare, even if you have to mill the intake side too. But seriously, cc the heads and see where you are if you are going to keep the stock ones.
And if you go with those Magnum heads and the 62 cc chambers, all of that head flow as you will get OOTB, from Indy RHS's or Edelbrocks, will be all usable for you. Those heads will get you to around 9.2:1 SCR on the present pistons.... not bad.