Well apparently you haven't tried to rebuild a 351+ V6 before the 4.56" and larger pistons all have to be custom made. Lots of piston options for 4.25" bores. Sounds like either way to go your dealing with "custom made parts"
Now when you figure in diesels are large torque engines most designed with a larger stroke NOT larger bore ! I also feel my idea will net a much better power to weight ratio once completed.
Have to get custom pistons anyway so may as well increase the compression say 9.5:1 instead of 7.5 or 7.8:1 . . . Sorry if I wanted "cost effective" I would put a modern small block in it.
None the less its off topic here so I shall discontinue the conversation here and start a new one.
Mike