I never have seen a manual transaxle for a transverse V8 engine. Do those even exist?
That is one reason I would prefer using a high performance 6 cylinder engine. Getting a manual transmission Maxima isn't too difficult, the engine is tough, and the fanbase is huge. Some of late 3.0L and early 3.5L Maximas had a 6-speed manual.
The VR6 sounds even better to me, because they take up very little room, and if you have to custom build a car, that would give you more freedom in how you design the car. There are many VR6 cars on the road with a 5 speed manual, and a few select versions of the 24V Jetta and GTI had a 6-speed manual. I'm not sure if the 6 speed from the 24V engine will fit a 12V engine, but with such a huge fanbase, you are sure to find answers.