You are assuming the 4.2 comes with a vehicle .
Simple - cost ! My Mk8 has only done 41,000 and have a 4.2 rebuilt engine
to replace the 3.4 which must have been an anchor in another life.
The 4.2 never came with a box so why spend money when I may have an option of
using the one I have ??
Thanking you in anticipation.
Physically, the engines are the same size, and about the same weight, I would estimate, but this will still be a major job, so are you ready for that ? Might be better to find a BW box that fits your Series 3 engine, a Model 65 or 66 will fit and maybe an earlier box that went on the Series 1 cars. There must be a few around still. That way the links to the rest of the car will be less daunting, (fuel, coolant, electrics etc).