We looked at buying an extra cab, I think the spot bolt the anchor point to had been added in, there was a plate on the outside of the cab just below the window with bolts through to another plate on the inside. If there isn't something already there it can definitely be done. I spoke to engineer in echuca and he told me it wasn't a problem to have one in there legally. Best look into that yourself also though.
Saying that there wouldn't be much room in there for one, seems barely enough room in the dual cabs for one, ours had to go on the passenger side as I'm 6"2 and it wouldn't fit behind the drivers seats when I'm driving.
Education is what you get when you read the fine print, experience is what you get when you don't.