Firstly ,, Phil is spot on , different mountings ,
the suggestion re swapping bases has some merit , but it should be noted the cut of the skin or squab is not the same , nor is the means of attachment etc . It is all very very similar but also very different , I am sure you could make it fit , but not sure how good it would look , Look at the bases and you will see what I mean .
I would look at developting an adaptor . Perhaps a flat or angle that you bolt to the rear mount bracket that incorporated mounting holes for the later seats , this is mere thinking aloud and would require a bit of trial and error . You would also need to rework the front mount holes again with an adaptor or a Replacement cross rail welded in ...
