What you lifting the enginge with? It's normally easier to get the enging in on the mount, have it angled down, put something under it so it's not just held by the mount, remove the box cradle and drive shafts idealy but can do it without taking them out and do it that way as its easier to giggle the box about to get it on. Also check the clutch plate is 100% lined up as if it's out a tiny bit the box won't go on. Once on just either jack or lift up the box to the right height and bolt the cradle back up