If you're serious about it, speak to an auto engineer - it will need to be engineered anyway to pass rego.
The guidelines for modifying your car in NSW are here:
https://www.rms.nsw.gov.au/documents...ifications.pdf
Each state is different, but for all of them you apply to modify the vehicle before you start, telling them exactly what you intend to modify and they'll give you a list of documents you must produce and tests you must do to pass rego.
Looking at the body modification section from the document above & given you'd need to add a bulkhead of some sort to close in the back of the cab, then it would need certification. If you're not radically altering the chassis structure of the car as it's already reinforced it might be relatively straightforward to get passed with an engineers report but it's best to ask an engineer directly what they think.