Several issues make it impractical, even for the most extreme builds.
1) A better 12 Port head already exists over what you would gain from trying to cobble V8 heads together.
2) Drilling the block to accept the V8 head bolt pattern weakens the block too much, and has been the cause of engine failures by those who have done it at that level.
3) The word "custom" usually doubles or triples the cost because all other associated components like pistons, intake, headers, valve cover and other specialty parts needed to make the V8 head fit the 6 cylinder block will also be required to be "custom". "Custom" = $$$$$$
4) See #1
