Reason for fitment is that it's a stronger box than our BC box's, larger bearings, much more ribbed bell housing, which all helps prevent shaft movement under high torque loads resulting in broken teeth and snapped shafts
It also has a built in hydraulic release bearing so no more heavy pedal when running a proper clutch or snapping cables

Using a couple of spare blocks I had, a spare BC box, and my Ka box I offered it up....
The gearbox bolts up fine, although it does have one bolt hole missing compared to a BC box (upper left, see pics).
The cradle also bolts up to the IB5 fine, but it is tilted up at the back compared to the BC box, this would result in the engine tilting backwards at quite an angle if left as is.



Back half of the IB5 casing is almost identical to the MK3 Escort BC box's, but the front half is very different.


How it should sit...


How it does sit...


How it sits now...








Extended the slot in the cradle on the left hand side about 15mm to allow the mount to sit lower, ground back left hand bracket and re-drilled lower, re-drilled right hand bracket.
Looks close enough to me

After fitment...
Had to grind a hole bigger in the cradle as the box was sitting on it and vibrating quite a bit, elongated the engine mount top hole to allow it to rotate a fraction.

May have gone a bit bigger than needed but I did it in situ lol
Easier way may be to grind a bit off the gearbox first but I couldnt be arsed to take it out again.