Historically the cars were very succesfull with the 4 and the 6. IIRC they even mounted an 8 cyl. but it was pretty rare.
FRAN,
Is there a huge dimensional difference between the VW F4 and a Porsche F6?
The Subaru option sounds great too but would you be able to keep it turbocharged and keep it intercooled. Based on the frontal area and shape of the 904 where would you mount the intercooler. I actually like the subaru option alot because of availability, modern engine management, driveability, reliability, sound, size, and many other parameters.
ON the other hand the sound and power of a stroked NA Porsche 6 would make me consider selling my soul to the devil......
To answer your question, yes I would pay more for the option of a F6