What Curtis said,
and they cannot be welded/repaired if they are broken once they are filled with block filler or bolt set cement.
Have i ever tried this? (nope) not on a set of heads. I've run solid as cast Brodix's on blown alcohol big blk. with a Rodack blk.
I would advise against anyone filling the heads for blown or otherwise. With a good set of alum heads, i'm talking 1/2 min. deck you can run 30 PSI wirh a blower and they will live.
JMO
Zip.