Assuming a .90ish delta leap and ignoring the other Greeks which shouldn't be too active, you'd gain .9 x 5 x 100 = $450 on the long call.

The short normally wouldn't be exercised pre-expiration day unless SPY is going ex dividend and the div > extrinsic left on the contract, but let's assume someone does exercise it. You are now short -100 shares at 257 that you need to cover. Assuming you don't have the shares to cover this position, your broker will most likely liquidate at the market value, which is a piece of information you are missing in the example. Also, and option needs to be ITM not ATM to be exercised. Let's say that spot is 258, so you are forced to sell at 257 and buy back at 258 losing $100. However, you collected $300 in premium so you are net +$200 on your short still.

Total PnL is 450 + 200 = 650. Covered calls are a bullish position and getting called away or exercised is not a bad thing.