In terms of hot first halves, the 1970s come in with a very big bang and go out with less than a whimper. (That will become more self-evident shortly...)
That's a total of twelve hot first halves over two years (seven in '70 and five in '71). Flukish hot spells were all over the NL in the first half of 1970, fueled in part by an early home run spike. Three hitters--Bernie Carbo, Dick Dietz and Jim Hickman--performed a good bit above their respective pay grades in the first half of that year, joined by Rico Carty (who kept it up all year and led the NL in hitting) and Tony Perez (who finished third in the MVP voting). Harmon Killebrew and Boog Powell jump-started their teams (Twins, O's) to their second consecutive rendezvous in the newly-minted post-season playoffs, with the same result as in 1969.
In 1971, Willie Stargell set a new April record for HRs (11) and matched that total in June as he helped the Pirates blast off to a blistering start reminiscent of the 2022 Yankees. (They had a similar mid-season swoon, but recovered to win the pennant and the World Series.) Over in the AL, Tony Oliva and Bobby Murcer didn't quite match their first half totals in the balance of the season, but they wound up 1-2 in the batting race at the end of the year.
Carlton Fisk's 40-game hot streak in '72 from June 10th to the (very late) All-Star Break (.358/.449/.754) captured many an imagination and propelled him onto the AL All-Star squad for the first of what eventually became eleven appearances in the "midsummer classic." He even managed to steal some of Dick Allen's thunder, at least for a little while. Over in the NL, 21-year old Cesar Cedeño looked as though he was going to become the game's next reigning superstar: it didn't quite work out that way. Billy Williams had his career year for the Cubs, who continued to go nowhere.
The following year, Willie Stargell tried mightily to lift the floundering Pirates (reeling from the tragic loss of Roberto Clemente), eventually leading the league in homers, RBI, doubles, SLG, OPS and OPS+. It wasn't enough: the Pirates finished 80-82.
No first-half hellion in '74, but Fred Lynn and Joe Morgan made headlines in '75, and their two teams would meet in one of the most memorable World Series of all time. Morgan was even better in '76, as his Cincinnati Reds became the last NL team to win back-to-back World Series. Rod Carew's great start in '77 was the only time during the decade that a Top 300 first half featured a player who hit less than 10 homers in his half-season slice.
And, as they say, that's that...there were no first-half Top 300 heroes in either 1978 or 1979, which leaves us with a total of just 22 first halves crossing that magic barrier for the entire decade. Within that group, there were no .400+ BAs, and only five .350+ BAs. Likewise, no .500+ OBPs, and seven .450+ OBPs. We did have one .700+ SLG (Stargell in '71), and a total of six .650+ SLGs (sixteen hitters had a SLG of .600 or higher). In homers, Stargell had both of the 30+ half-seasons, and there were 11 instances of players hitting 20+ HRs during their Top 300 first halves.