The biggest difference is that an SPA is the sale of all shares, and an APA is the sale of selected assets. Therefore, they are both different transactions and have different procedures.
A penalty may be imposed for failure to file Form 8804 when due (including extensions). The penalty for not filing Form 8804 when due is usually 5% of the unpaid tax for each month or part of a month the return is late, but not more than 25% of the unpaid tax.
First and foremost, it is typically the buyer's responsibility — not yours as the seller — to draft the Definitive Agreement. This will not begin until both the buyer and the seller sign a Letter of Intent indicating their intention to buy/sell the business.