Making power out of the 318

Yes you could but how available are 273 heads and how much valve can you put in them?
Port flow volume is just part of the picture. Flow velocity is also important and where in the port you get that velocity. Flow volume and flow rate combine to generate port energy. This is the important number.

I have actually run 360 heads on a 273 and 318's. I much prefer them over 318 heads.