It's a popular question...just not a popular project. I've seen it done in a few cars but I don't know the owners of any of those cars. I don't think anyone on this forum has done it though. There was a rabbit pickup from england that had an mk3 dash conversion, but thats going way back.
From what I hear the hardest part is getting the guages and heat controls to work(corectly).
What year is your car. I know that pre 90 cars are harder to do. Not sure why though. Pretty sure it's wiring. I hate to say this but the vortex might get you answers quicker. There are a ton of peeps on there with the swap done. good luck. I'm pretty sure you'll need to weld in the brace form a mk3 for it to blt in.