The Mk3 dash pretty much bolts straight in. 4 bolts at the top, two at the bottom and the one on the exhaust tunnel. There's two more on the end of each side but we left these when fitting mine as there was nothing to bolt them to and the dash was already in tight.
The only
problem you'll have is the dash won't line up with the door cards, and you'll have to locate your electric window and mirror switches (assuming you have them) in the new dash as the Mk3's had them in the door's. New a-pillar trims will also be needed. Not sure about wiring for the stereo, heater, light switches etc... either as we used the full Mk3 wiring loom with mine.
They do go in though