Talk to Tim Ayres at New Elms.
They now have Rutherford Engineering parts. They will know whether there is a kit available for the earlier cars.
Please post up any results, there's quite a few older cars here.
Had it done on my 1989 4/4, which I believe is identical to yours, using Rutherford parts. There are (or where 5 years ago) two option avaiable. One needed to make a slight cut in the woodwork, but nothing drastic. As Graham has said, give Tim Ayres a call.