I think you would struggle to authenticate the van on Ebay was one of the ones from the trial. I've serviced on rallies for the best part of 20 years have never had to list the VIN# of the service vehicle on any entry form or paperwork for the rally organisers. Most info ever recorded was a rego # and make, model and colour of the service vehicle. Given the the promoter (Stuart McLeod) of the Repco Trial was killed in a bomb blast in the mid 80's trying to get hold of any paperwork would be a challenge. My brother who navigates had a bad habbit of listing the service crews address as the workshop address of where the rally car was prepared so I would be worried about quality of any info as well.
The other issue is that other than a few died in the wool Holden enthusiast not many rally people are interested in the HDT vans. The vans to beat in the late 70's early 80's where the works Datsun E20 vans. The Datsun rally team guys shoehorned a 6 cylinder motor out of a 240-260Z rally car with a close ratio box and 4 wheel disc brakes.