Three hundred fifty out of a record 2434 applicants received their letters of acceptance to the Radcliffe Class of 1971 yesterday.

The applicants -- up a staggering 17.5 per cent over last year -- were of "higher quality than ever before," Margaret W. Stimpson, Dean of Admissions, said yesterday. But the key to next year's freshman class, she noted, is "diversity of backgrounds and interests."

Those accepted include a record number of students offered financial aid - 104 compared to 86 last year -- and the total award rose to $179,650 from $146,290.

About 15 Negroes, more than ever before, were accepted. Radcliffe was the only Seven College Conference school which had a "significant" increase in applications from Negroes this year; the total number of Negro applicants to the seven colleges declined.

Seventy per cent of the new freshman class attends public school, and 30 per cent private. Last year the ratio was closer to 65-35, and Dean Stimpson expects that same proportion to develop again as final acceptances come in.

The class includes 43 National Merit and National Achievement Scholars, about half of those who applied. Thirty-one sisters of Cliffies (about half who applied) and 30 daughters of alumnae were admitted.

The largest number, 107, come from the middle Atlantic states -- New York, New Jersey, and Penn. Ninety-four are from New England, 56 from the South; 44 from the mid-west; and 40 from the far west.

The freshman class has 300 places, and Radcliffe based its 350 admissions on the number of girls who have accepted in the past. "If all 350 came, I don't know what we would do," Dean Stimpson commented. The number who accept will determine the fate of the more than 200 transfer student applications and the small waiting list.