There are differences between the diffs.
I have two Mk1's, an F reg that I use regularly and an E reg that I bought for spares, the front diff casings are different shapes although both have the same ratio.
The E reg one did eventually fit into the F reg car but I had to use it's matching mounts and crossmember and then modify the bolt holes to line up :roll:
In short, do a really good comparison check first