There are ways of doing it if one is really determined.
One way is to cut the rear apron and have the engine stick out the back. I know you asked how much and I have only seen this once with a Baja. Since the engine area was open with the lid and fenders non-stock, it would be hard for me to guess off of a ten-year old memory. If forced to guess, I would say six inches. It did strike me that it was quite a bit and it would look extremely abnormal on a regular bug.
The other I have seen was even longer ago. This person moved everything forward. It required
a lot of fabricating as well as the removal of the rear seat. He did a nice job but I got the same sense then as I did at BugJam last November when looking at bugs with V-8's in the front -- a lot of work for the wow, but how practical is it reallly if one wanted to genuinely drive it?
A quick search on the latter netted me one pic.
I didn't see any blow-by-blow descriptions on how it was done but it seems Kennedy Engineering makes adaptor plates to mate trannies to engine.
Then, there's another novel idea I came across when searching. Instead of adapting the engine to a bug chassis and body, he adapted a bug body to a 911 chassis. From there, the engine was easier to work with and he was able to keep other Porsche things like the electricals. Here's the link:
Project 50/50