Is a Sandman still a Sandman if it's tags are gone???

no.. its just another van and there is no real way to prove that its a genuine unless you have the original purchase papers and log books.

Is a Sandman still a Sandman if the chassis doesn't match???

no ...its identity has been changed and therefore does not correspond numbers wise with its main tag that has the chassis number on it...see question one answer for expansion..

Is a Sandman still a Sandman if it has had a body swap???

yes if it retains its tags and chassis numbers and they all match..

--- Updated ---

just to expand on what i have said above...
die hard restorers that go to other forums and have seen similar discussions may believe that removing tags and putting them on other bodies is wrong but the fact that the vehicle has a removable body from an independant chassis changes the game a bit..
you could buy a chassis from holden and back then the beaters stamped the original chassis number in it so there was no grief in reregistration...
you could also buy a body shell which would come with tags stating it was a gm replacement but again these were launched for ease of reregistration or whatever.
just a big meccano set...:Flower: