What thickness gasket and how much should I deck an LT1 head if I want to raise the compression to a 12:1? Everything is stock...

 

Use the Impala gaskets, they're 0.029" compared to 0.049" stock. Generally for SBC removing 0.007" from the head is 1 cc. If your stock chambers are 55 cc then you'll need to go down to 48 cc, so you'll have to remove 0.049" from your heads and use the thinner gasket.
You can also deck the block for a thighter quench, and shave less from the heads.