Build the firewall perpendicular to the thrust line and use shims (washers) under the engine mount to adjust the thrust. I'd start out with a couple degrees of both, maybe even 3 down and 2 right. You've got a big round hole in the cowl to stick the crank shaft out of, so you've got plenty of room for adjusting these lines. Maybe offset the engine mount upwards and to the left slightly to compensate for the down and right. I'd fly and see how things work, before installing a mock rotory.

It's interesting, thrustlines used to be discussed much more then they are now. Maybe this is because in the world of ARFs, we don't have to think, anymore!