All HQ sedan, wagon, coupe or Statesman use the same chassis, not just Kingswood or Monaro in these body styles. It isn't until later into HX-HZ where some of them start to get shorter at the back and use less mounts, but even so any HQ-WB passenger chassis will fit, just some of them won't use the rearmost mounts, WB you can't get the front bar front bolt in on either side (so only 2 out of 3 fit). From mid 1977 (during HX) the rails change a bit for RTS suspension geometry, and HZ-WB rails use thinner steel (possibly from mid 1977 HX as well). From this time the chassis are drilled for fitment of strengthening bars too which are a good idea to help minimize cracking.

The other issue of course is the chassis number, so by changing rails you are changing the legal/registration identity of the vehicle.

Your best bet to minimize cracking in a replacement chassis is to use rubber bushes, and use original specification springs (not not really heavy and/or lowered ones). The best way to lower them is to use drop spindles and keep the original suspension height. And fit the HZ-WB bars.