Nimble, I'm glad that helped. However, engines are notoriously expensive for the boxster. There has been an Audi conversion and a Subaru conversion, but from what I hear it is extremely tedious and time-consuming. However, for the motorsport aficionado, engine swaps are very common for the dedicated. Based on what you have to say, I'd say an Audi V8 would be your best bet, as Audi and Porsche are practically sister companies. A guy did use an engine from an A8 but it looked messy with wires and two gear selector knobs in place!
You may have already seen it but here is the video in case you haven't:
Porsche Boxster V8 80% complete. - YouTube