Much as Fastburn said.
For my rear wheel arches I measured where the axle was when the wheelbase was set short, and when it was set long, then basically score the two arches and where they meet in the middle at the top (there will be a slight V shape left due to the two separate circles not being lined up). Just trim this out with a knife.
Make sure when the body is pushed inwards past the tyre that it will not catch on it as this will cost a lot of time if a marshall has to sort it.
Xray T4'16, T4'14 (Wet Car)
Xray X12 2016
wlrc.co.uk (West London Racing Centre)