HaRav Heschel was truly a Gadol BaTorah. He never abrogated Halacha and the only reason why he joined JTS is because he thought it would become something similar to what YU is now. He thought potentially it would adhere to Halacha and reach out to the Jewish ignorant masses in a modern style. It is to bad he was wrong. Its interesting that his mechitza beis medresh in JTS still exists.
My father who has smicha from Rav Aharon Soliveitchic has many of HaRav Heschels sfarim.