That will work with one "gotcha". All front driveshafts are the same length, and the bearings are the same, so it will fit OK.
The gotcha is that Alfa increased the diameter of the four flange bolts (both F and R) from 8mm to 9mm between the 1750 and 2L series. So the holes at the back of your front driveshaft flange will be 9mm, while those at the front of your rear driveshaft will be 8mm.
Using 8mm bolts in 9mm holes is not a good practice. A better solution would be to use a U-joint flange from a 2L driveshaft at the front of your 1300 Junior rear shaft.