I wouldn't worry too much about the rails. If it was a Sandman it was a 253 auto HX and thus will never set the world on fire value wise. I'd just build it up to look original externally but do what you want underneath. Being HX you do not suffer from any additional pollution equipment going to HZ rego (which it will be by the rails), and you gain much improved ride and handling with the HZ suspension. It would look fantastic painted in its original colour and trim combo. I'd stick an EFI 304 and 4L60 into it, run 15x7 GTS rims and standard ride height. Will make for a very impressive vehicle with or without stripes. Dont hesitate to fit integrated A/C and power steer either.