...is the only pitcher in modern baseball history (i.e., post-1920) to lead the major leagues in wins over a ten-year span and lead his own league in ERA and be rejected by the Hall of Fame?

Yes, it's true. In fact, you can lead one of the major leagues in wins, ERA and strikeouts over a ten-year span and get about the same number of votes for the Hall of Fame as the great Bobo Newsom. Ron Guidry is proof of that astounding fact.

Ron Guidry won 163 games from 1977 to 1986, had a 3.23 ERA and 1623 strikeouts.

Here are the major league leaders from 1977 to 1986 in wins, ERA and strikeouts:

Wins
Ron Guidry 163
Steve Carlton 155
Phil Niekro 149
Joe Niekro 145
Jack Morris 144

ERA (min. 1600 innings)
Steve Rogers 3.10
Steve Carlton 3.13
John Candelaria 3.14
Nolan Ryan 3.19
Ron Guidry 3.23

Strikeouts
Nolan Ryan 2192
Steve Carlton 1929
Ron Guidry 1623
Phil Niekro 1594
Bert Blyleven 1544

The astute reader will note that all four pitchers ahead of Guidry in the ERA list were N.L. pitchers (actually, Candelaria and Ryan both pitched in the A.L. as well, but spent the bulk of the ten-year period in the N.L.). National League ERAs naturally tend to be lower (because of the absence of the designated hitter), and therefore Guidry's achievement is all the more impressive (in fact, his ERA+ was lower than the N.L. ERA+s of any of the four pitchers ahead of him on the above ERA list).

Who have been the other pitchers to lead the major leagues in wins and their own league in ERA over a decade span? I'm glad you asked. There have been ten. Here's the list: Lefty Grove, Carl Hubbell, Hal Newhouser, Warren Spahn, Juan Marichal, Tom Seaver, Jim Palmer, Steve Carlton, Roger Clemens and Greg Maddux. The first eight are already Hall of Famers; the last two, Clemens and Maddux, will join them the moment the votes are tabulated in their first year of eligibility (unless Clemens gets the McGwire treatment).

Strangely, the eleventh man to accomplish this feat not only failed to make the Hall of Fame, he never even came close. Which raises the question: what did Ron Guidry ever do to offend those who cast their ballots for the Hall of Fame?