1.25 or so HP/C.I. from a 390

I think you've got your math backward on the 440 and 340. 375hp/440ci=0.85hp/ci. 275hp/340ci=0.81. That aside, there are alot of 360s running 425-450hp. A 390 should be able to get there easy with the correct combo.

How is the math incorrect? To the best of my knowledge, it's not a percentage measurement.

Or is it?