The tongue extender is highly illegal Kev, your best shot is to work on the hitch and get it free, might take some work but that's the best way.
Then if the box section behind is long enough drill a second hole through that will give you the necessary length required or get a new assembly either made or bought.
I have never seen a hitch so badly rusted in that it wont come out or be not too rusted to be safe.......you need to make the decision as to where it is structurally sound or not after removing it.