They changed from short to long shifters in the Feb/Mar timeframe as best I can tell. If you look at the collections of Sunbeam magazine articles by mid to late April cars were appearing with long shifters and if you figure that it takes a couple months from the actual photos and raod tests to publication, then this makes sense.
The convertible top covers are the same as a series IV Alpine so you can probably find a set if you check for guys advertizing as breaking Alpines.