Americans are feeling a little better about themselves and the state of their nation, according to a new "Gross National Spirit" index developed by the American Enterprise Institute, a Washington think tank. The index tracked U.S. public opinion on six specific attitude questions from 1974 through 1981 and showed that the American spirt was lower last year than right after the Watergate scandal but is on the upswing now. According to its principal developers, Ben Wattenberg and Everett C. Ladd, the index -- with a theoretical top score of 2,400 -- stood at 1,026 in the Watergate year of 1974, rose to 1,162 and 1,274 in 1975 and 1976 and peaked at 1,404 in 1977. It dropped to 1,188 in 1978, 1,047 in 1979 and 956 last year. But this year, the index rose to 1,275, the second highest level in the 8-year period. Wattenberg is co-editor of Public Opinion magazine, published by the institute, a private organization regarded as moderately conservative. Ladd is director of the Roper Center, a public opinion polling organization. To achieve what they called "a new index designed to assess the overall national spirit -- how Americans feel about themselves and their country," Wattenberg and Ladd combined a battery of six questions that have been asked by national polling organizations for some years. Each question was assigned a maximum positive score of 400. The six questions: Taken all together, how would you say things are these days -- would you say that you are very happy, pretty happy or not too happy? We are interested in how people are getting along financially these days. So far as you and your family are concerned, would you say that you are pretty well satisfied with your present financial situation, more or less satisfied or not satisfied at all? How do you feel that things are going in the country these days -- very well, fairly well, pretty badly or very badly? Do you have a lot of confidence, some confidence or no real confidence that in a few years from now our country will be strong and prosperous? Now turning to business conditions in the country as a whole -- do you think that during the next 12 months we'll have good times financially or bad times or what? Do you approve or disapprove of the way (name of incumbent) is handling his job as president?