On the 1976 results page, Dave's "What If?" says...
From what I can tell, a much smaller swing is actually necessary: Ohio and, instead of Wisconsin, Hawaii. If Ford had won both Ohio and Hawaii, he would have carried the Electoral College 270-268. Carter only carried Hawaii with 7,372 more votes than Ford, so a swing of just 3,687 votes in Hawaii, plus 5,559 votes in Ohio would have flipped the results.
Part of the problem is that one Ford elector in real-life voted for Reagan. Had he done so in this alternate scenario, then the EC would have deadlocked 269-268-1 and the election would have gone to the House. But it's doubtful that any of Ford's electors would have defected if there was any room for error.
If that is the concern, then a swing of Ohio and Mississippi would still have gotten Ford a victory and with a smaller swing than Wisconsin. Carter only won Mississippi by 14,463 votes, meaning a swing of 7,232 votes in Mississippi plus 5,559 in Ohio would have given Ford the election.
So why is the What-If for 1976 based on a swing of Ohio and Wisconsin?