Edwin,
Oops After writing the following I noticed you have no plans? You could order a set of plans.

With my 1/4 scale cub what I did was I laid the engine on it's side with the motor mount attached on the plans so the prop hub was lined up with the prop hub on the plans, then I marked where the engine mount was for the placement of the firewall, which happened to be 2" back in my case.

Patrick