The index has been on a breathtaking run, but some S&P 500 stocks still have big upside, according to Wall Street.After notching another all-time high earlier this month, the S&P 500 index has been taking a breather. The widely followed benchmark index -- composed of 500 of the largest U.S. companies -- has hit new highs 38 times thus far in 2024, so it's earned a break.Some investors believe this pause could be a sign the rally is on its last legs, but others on Wall Street are unconcerned. Among the bulls ...