The chassis is the legal and ADR compliance identity of the vehicle. So a HQ with a WB chassis becomes a WB (AWB, BWB, CWB etc). Therefore to the letter of the law the vehicle must comply with all ADR's applicable to the vehicle the chassis came from. Which means more than pollution requirements. Technically you'd have to get the HQ body modified and Engineered for retractable seat belts, you'd have to change blinkers, comply with black wipers, probably change at least the dash warning lights for brakes. It may even mean changing the dash if there is any ADR's that mean you cannot have a steel dash in front of you.

This is what makes a joke of the stupid laws about changing ID plates - in theory the way to do the chassis change properly is to use the ID plates off the vehicle the chassis came from, as those plates tell you the ADR's the vehicle complies with.