I don't think that bellhousing suits a W5X box (W55,56,57,58,59).
From things I have read on rollaclub, it actually suits a W50 or W40 box (steel case)
W50's can be found in just about anything manual pre-80's, they are also stronger than K50's and T50's again.
Wreckers tend to sell them for a bit though, typically its because the holden boys are stupid and seem to think that because the casing of the box is steel it must be better than the alloy case W5X's.
Don't take my word for the gospel, but go have a look on and you'll find out more info.