Pulling the speedo cable angle drive from the gearbox is a cinch, just undo the large nut that holds it on. The 'don't want to do' you refer to is the 10mm bolt below the angle drive, which you should not undo as it allows the internal speedo drive shaft to drop inside the ZF.
I've just been through this, #9178 was reading 60mph when I was traveliing 75! Sure Officer but I was only doing 60 is not a valid excuse apparently
.There are two types of ZF angle drives Veglia-Borletti and VDO each with different ratios 1:1 and 1.1:1 (4 choices). Also the speedo internal ratios depend on whether it as Euro (kph) speedo or US mph speedo. The speedo is marked on the back either '1610 giro' revs per mile or '1210 giro' revolutions per mile. The 1210 is for the Euro speedo and goes with the 1.1:1 angle drive. You have to find the right combination. My car was a Euro import and converted from km/h to a mph face, I changed the ZF angle drive and it now reads accurate to within 4 mph at 80 mph.
Good luck
Julian