O great mountain … become a plain: This phrase is a metaphor used in both the Old Testament (see Isaiah 40:4; 49:11) and the New Testament (see Matthew 17:20; 1 Corinthians 13:2). It means that faith in God helps people overcome difficult challenges.
The capstone or headstone, which is the final stone, completes a stone wall or building (see Psalm 118:22).
Grace, grace to it! is a prayer asking for God's favor and blessing on the new temple.