On Dave_iow's Golf we've fitted the G4 shifter, cables and the shift tower on the Golf4 gearbox.
We had to trim a lot off the G4 shifter mech, but that seemed to be OK. The shifter mech is bolted up at the front on the two studs at the front. At the back we made a pair of tabs and bolted through the tunnel to 'pinch' the back of the shift mech up.
We had to trim various bits of bracketry out the back of the tunnel, but we didn't need to take a hammer to the tunnel at all.