Chassis is the same between models of the same month, only differs between series (say HQ to HX) or same series but built many months apart (eg 11/76 HX vs 8/77 HX). So model wise, a HX Kingswood ute chassis will be identical to a HX panel van chassis as long as it is from the same plant and month.
So if you use a HX ute chassis in a HX van, the vehicle stays as a HX but the identity of the vehicle is always by the chassis, the rest of the tags mean nothing to vehicle identity (from a legal/registration/ownership/insurance perspective). Taking this to the extreme, imagine you have a 1971 HQ Monaro(337) shell. You want to build a nice driving car out of it, so you fit a set of HZ station wagon rails to it (for the RTS). The vehicle gets registered as a HZ coupe, regardless of what the ID plates show. A vehicle registration history search shows that the vehicle was originally an 5044cc HZ Station Wagon. So effectively as far as the law is concerned you have rebodied a station wagon, not re-chassied a coupe. Also the letter of the law requires you to comply with HZ ADR's, which means ADR27A engine, intrusion bar doors, retractable seat belts, amber lenses, black wipers, safety (JJ) rims etc etc.
Clear as mud! This and many reasons is why the whole tag nazi BS is stupid and ridiculous. So concours restore the complete original matching numbers cars, and modify the others. There is no good reason to try to restore a rusted to hell Sandman body if it doesn't have the original rails with it, use a donor body and enjoy!