I think it's quite certain he would have won more Electoral votes than McGovern's 17. Wallace wins Alabama and several other Southern states.
I agree that Wallace never could have won the Democratic Party nomination. But, hypothetically, if he did, you would certainly see a liberal third-party candidate running. Liberals, many moderates, and most non-Southern Democrats would be unhappy with a Nixon-Wallace choice. Someone like McGovern would probably run -- not to win, but just to give the non-Wallace Democrats a place to go.
Wallace takes votes from Nixon, allowing McGovern to win some states he wouldn't win in a head-to-head Nixon vs. McGovern matchup. Wallace wins more states than he did in 1968. As the official Democratic nominee, he's stronger than he was in 1968. He wins Alabama, Georgia, Mississippi, Louisiana, and Arkansas, plus a few new states -- like Tennessee, the Carolinas, maybe Florida.
It's possible no one wins the Electoral College, and Congress decides the presidency.