Breaching Dragonstorm
When this enchantment enters, exile cards from the top of your library until you exile a nonland card. You may cast it without paying its mana cost if that spell's mana value is 8 or less. If you don't, put that card into your hand. When a Dragon you control enters, return this enchantment to its owner's hand.
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Official rulings (3) · Scryfall
Since you are using an alternative cost to cast the spell, you can’t pay any other alternative costs. You can, however, pay additional costs, such as kicker costs. If the card has any mandatory additional costs, you must pay those.
If the spell you cast has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.
If you choose to cast the exiled card, you do so while Breaching Dragonstorm’s first ability is resolving and still on the stack. You can’t wait to cast it later in the turn. Timing restrictions based on the card’s type are ignored.