There never was much of a distinguishment between the Mk.I Rhino and the STC Rhino. As for making the older version... I'm kind of curious as to why you'd want to make one of the old clown cars. The MkII kit at least actually fits ten models if you take them off their bases (I've tried-- two in the front, eight in the passenger compartment) and thus is actually more to scale than not.
You can certainly get the old Rhino look with the new model, though. Pretty much, all you need is a Chaos Rhino box (as it has a spiky ram on the accessory sprue) and some plasticard to make the round doors out of. From there, it's just a question of mounting a single bolter on each cupola.