No I built the original fuselage as per plans but with OS 40FS the Carb is at the back which pushes the engine forward. The cowl overlap would originally have been about 40mm now it is 20mm - which is fine for attaching the to the fuselage.

When you get to this stage of the build just mount your engine on the engine mounts and see where the prop will sit then adjust the firewall back if necessary.