erm pedal box no, slave cyl maybe! I think a rado/passat slave cylinder would be more suitable. the pedal boxes are a little different as the brake servo mountings arnt the same in a mk3... You might be able to use the entire pedal box, servo and slave cylinder from a passat/rado though? I dont really know enough about the mk3 chassis to say with confidence if the mounting points are the same.
To be safe I'd try to get a MK3 VR6/ABF pedal box, servo, slave etc then you know it'll fit.