On my mk4 I'm running an IHI vf34 BB turbo, was able to mount it on the OEM(heavily ported) manifold with a couple of adjustments. What was really needed to alter was the downpipe(as the VF has a 5-bolt pattern DP)..
Everything is doable like Browett mentioned, but some aer easier than others.
I believe I saw an adapter for these turbos a few years back, so that would enable it to mount it directly onto the OEM manifold.
But the OEM manifold needs to be ported to get the most benefit out of the turbo as the OEM runners and transition limits exhaust flow and causes too much back-pressure..