Instead of a single overall score, the 1978 GED likely reported results separately for each of the five subject areas tested (likely similar to later tests focusing on reading, writing, mathematics, science, and social studies). A "high score" would have meant a strong performance in each of those individual sections, meeting or exceeding the passing standard for each. The passing standard itself varied over time and wasn't a fixed, publicly known number.
To put it another way, a "high score" on the 1978 GED meant simply performing exceptionally well on all five subject tests, achieving a score high enough to be considered a strong demonstration of high school equivalency according to the standards of the time. Without access to the specific scoring rubrics and percentile data from 1978, we can't assign a definitive numerical value to a "high score."