I've seen this done in 924/944s. I've also heard the Audi V8 mates with the Boxster/Passat transmission.

Would the Audi inline five mount to the Boxster transaxle? Obv. motor mounts would be custom, but is there space for the engine?

 

I would think height clearance would be a issue

 

I know that engine very well. It is too long, too tall and while the engine would bolt up to the five speed Boxster transmission, the six speed would need bellhousing modification. Considering the work that it would take to make that old engine fit and give adequate power, you would find that there are better options.