You can't do much with the block to lower compression. Pontiac managed the CR in the cylinder head.

You're seeing the light and planning your build then finding the parts, rather than the other way around.

400 compression is a bit of an issue because few of the factory heads are true bolt-ons that will result in the right compression for today's pump gas. Most of the available heads will either need to be surfaced to reduce the combustion chamber volume, or you'll have to get dished pistons from any of the many vendors who carry them.

Are the heads you have 6X-4 or 6X-8? (The -4 or -8 ID is stamped on a boss below the rocker cover rail between the two left-side spark plug holes.)