Sorry, but another drive shaft question folks!

[my250gt]

I have bought a replacement drive shaft but it is the shorter one for the BTR diff. My car has the GKN diff and the old shaft is about 20mm longer. Q. Will the shorter shaft fit and work ok or do I need the longer one for sure?
I ask because I am told the earlier fitment (longer) shafts work when swapping from GKN to BTR diff??
Thanks, Phil.