I don't know enough on the manifold and PS..sorry. Look at Cempbell Enterprises too; they list the 526AP's still so must have some stock. The 285AP's will drop a gob of compression, with their lower compression height AND eyebrows.
Does the 340 come with heads? You don't want to try to use the 302 heads with the 340 block and pistons.... CR would be out of sight!