Yes and no. I see you are after the same type of thing as I am and I have been (still) doing a bit of work in the background.
I started with a packaging study of the frame, all in CAD of course, using known dimensions and few photos.
Here's what you'll get with the V8 BNS from an RS4 (or the lower revving BVJ/BAR/CAU) combined with an 01E
2071-BNS-01E.jpg
This BNS is now mated to a 0A2 (newer than 01E)
2071-BNS-0A2.jpg
This is a V8 BNS combined with an 0B3 (which was never actually manufactured, it just exists in literature, but you could possibly adapt an 0B4 shorter by removing centre diff somehow). As you can see it's super long.
2071-BNS-0B3.jpg
When I started looking into the 3.0TSFI (CAKA) engine more dimensionally, I think (>90% sure) that it fits to the older 0A3 or even the 01E trans. It will also fit easily (relatively) into the back of the 818, but my reasons for not going down this path have been more to do with the lack of FSI ECU's available, and the cost of the engines. I had one lined up ready to buy (with a hole in the side of the block, but that's another story), but on inspection, it had been sitting outside for a long time, and I just wasn't convinced it was even worth the cheap price they were asking.
As far as transmissions, the 0A2 (FWD)/0A3 (4WD) is the successor the 01E/01X, but I believe they have the same bell-housing bolt pattern. The newer ones are all aluminium.
So my research suggests:
B6 S4 used the V8 BBK with 0A3
B7 S4/RS4 used FSI version of the V8 (BNS/BAR), with 0A3
B8 S4 used 3.0TFSI (CAKA) with 0B4 (Manual), or 0B5 (Dual Clutch). These are the gearboxes with the front axle way up against the engine, actually in front of the clutch.
B8 RS4 used the same FSI V8 (BNS) with 0B5 (Dual Clutch)
Other info I have found is that the Cayman/Boxster used a 01X, and the Cayman/Boxster 'S' uses a 0A2.
The reason I don't think the newer transmissions (0B4 etc) will work is that the newer geometry (axle forward) means the gearbox is far too long out the back, or pushes the engine forward, resulting in crazy driveshaft angles. Not to mention that the axle output flanges are super wide making this even worse.
My personal conclusion from all of this:
Engine from a B6 S4 (BBK non-FSI) for ease of parts/supply, short length and price. Only negative is the chain tensioners will need updating to B7 spec
Transmission 6MT 0A2 from either diesel A6 or Cayman/Boxster
Hope this helps!