What is your HP to cube ratio?

I have been told that with my 416 I should only look to be around 600-630 HP b/c of the HP rule of thumb (1.5 hp per CI ) meaning 1.5x416=total hp?

However I have a buddy that has a 393 SB mopar with W8 heads that made 730hp.

I was hoping to break into the 700hp realm with this setup.