They weren't a Kingswood, the last Kingswood lux level coupe was a HQ Monaro (80337 and 80437). Only LS (Premier) and GTS in HJ and the standalone HX LE after that.

The firewall was changed as a HJ firewall is different to a HX in a few areas, and it must have been deemed better to replace the whole thing rather than weld up holes and move stuff.