The block you have is likely a "Stage II" block, sold through GM Performance Parts and never used on a production vehicle. The GNX had a standard Buick V6 turbo block - nothing special and certainly not a Stage II block. The Stage II blocks have much more meat, 4 bolt mains, and 6 bolts per around each cylinder in the head vs. 4. Stronger everywhere. See
http://www.gnttype.org/techarea/photoguideindex.html to check out the blocks and heads, plus some intakes.