I was thinking the same exact thing about the 25D heads. The valve/quench area might simply be too large for the diameter of the 18E bore, even if they can bolt on. Maybe 2.0L heads?
the bore difference between the 18e and the 22e is 9mm diameter . that would be 4.5mm radius, or about 0.177 inches, just shy if 3/16 inch.