Brett
I seriously looked at buying this truck but was put off by the transport costs to Western Australia. This GMC would be a wonderful exhibit when restored. Take your time and do some research into the truck and list what is missing and how much it will cost to replace. I spent quite some time looking and comparing the photos of the tanker and found that there was quite a bit missing. That does not mean that these parts cannot be found on the farm or surrounding area. Find out its ownership history and work your way back to see if parts are still lying around. If not you will have to source them from the US. Very expensive.
Good luck with the GMC and if you buy it, post heaps of photos.