This is from the RMS website:
A vehicle is classed as individually constructed if it has:
- A specially constructed chassis (non-production vehicle) or a manufactured replica chassis
- A widened or narrowed chassis, either in places or along the whole length of the chassis
- A production chassis which does not retain at least one of the original structural cross-members in the same place for that chassis
- A substantial change to the arrangement of the engine and driveline, such as engine moved from front to rear or to a mid-mounted position, or if the vehicle is changed from front wheel to rear wheel drive
- A monocoque construction, where the subframe structure has undergone significant structural change, such as removal and replacement of subframes with structures of a different design, or modification of the subframe rails
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It doesn't specifically mention shortening of the chassis, only widening or narrowing of the chassis.