I had the same issue with a 1/6 cub. I built the fuselage as per plans then when I mounted the engine, OS 40FS, I noted that the cowl was 3/4" farther forward than it was indicated on the plans. I measured 20mm back from each side of the current firewall and cut using a razor saw - in this way the relative thrust offsets were transferred. Then I drew around the cut section on 1/4" birch ply (not lite ply!) and made a new firewall to fit snugly into the opening of the fuselage. The cowl is such that in the was enough purchase on the cut back fuselage to properly mount the cowl and keep the propellor in the "scale position".

The important thing is to keep the propellor in the correct position and work back from there.