Tim, it's a while since I did this, but looking at my drawing/measurements made at the time, I removed 8mm. My measurements were: g/box front face to end of 1st motion shaft = 207mm; depth of spigot drilling in crank shaft = 27mm; first motion shaft protrudes 30mm beyond bell housing; bellhousing depth = 178mm; g/box front face to bottom of crank spigot drilling = 199mm. This resulted in 8mm needing to be removed, and seemed to work OK!
You can do it easily in situ with a cutting disk, just cover everything up esp the fuel lines as there loads of sparks. The metal is quite soft an cuts easily. Dont forget to put a bevel on the end to aid refitting to the engine.
Malcolm