Cheers, I just did some rough measurements on the two boxes in the shed, the standard 4 speed has the input shaft protrude ~16mm from the bell housing. The W5x box has the input shaft shorter than the bell housing by ~3mm and the bell housing it's self is ~ 161mm long.

If/ when I get to it I can take the bell housing to work and measure it on the flat table with a height gauge. Concentricity is more critical though.