The latest company mentioned to consider building a new high performance aluminum cylinder head for the GM 194/230/250/292 inline 6 was Classic Inlines.

http://www.classicinlines.com/

I last spoke with them about 6 months ago, when they stated a prototype head for the GM inline 6 was about a year away. The sales were less than expected to date for their new aluminum cylinder head for the Ford Inline 6 small block, 144/170/200/221/250 cid engine.