Jimmey, you need to know what you are asking.
When you say "diff centre" do you mean this whole bit:
Because that's normally known as the "third member"
WT and NT are interchangeable but not the same - as has been pointed out, ratios are different. WT - 3.7, NT 3.9. The driveshaft flange also has a different bolt size and pattern.
Or just this bit:
Which is the actual diff centre.
Rear WT and NT diff centres are the same.
The front is different - NT used the more desirable 4 spider gear centre (like a rear) whilst WT used a less desirable 2 spider gear centre.
Again, interchangeable, but not the same.
Steve.