The highest turnout for a national election in Scotland in the past fifty years seems to have been the February 1974 General Election, where over 78% of registered voters voted.
The turnout for the devolution referendum in March 1979 was 63.72%: 51.62% of those voted Yes to a Scottish Assembly, 48.38% voted No, a majority for Yes of 3.24%. But, according to the terms of that referendum, set down in 1978, the Assembly had to get over 40% of the electorate – there were 3,747,112 registered voters, so they needed at least 267,908 more votes for Yes to be allowed to win. 1,359,540 people were registered to vote and didn’t – the turnout was 63.72%, with only 0.13% rejected ballots.
The UK General Election in May 1979 got a turnout of 76.84% – that is, 532,198 more registered voters turned out to vote three months later than in the Assembly referendum. To win an Assembly under the 40% rule, the campaign would have had to get a higher turnout than average for 70s General Elections, and maintain its 51.62% share of the vote.