Two years ago I purchased a 963 which I run on a 1440. My dealer says some 1440's (2 cylinder) handle the head and some don't. My does with no problem. I had also inquired about adding another cylinder which is not an option on my combine. For what its worth, combine handles the 6 row head pretty good. In 2003 I shelled a field that yielded 215 bu_a.
i bought a 1440, it already had the 3rd cyl, put a 6-row head on it and it wouldn't lift it much over a foot an a half. never could get it to lift any higher, but have seen other 1440's lift 6-row heads fine. (IJIJIJ)