Kids' book where a girl trades her heart for a heart made of lead

I'm looking for a middle-grade novel written in the 70's or 80's where the heroine gives up her own heart and exchanges it for the lead (metal) heart of a male character. As a result, he changes from a cold, cruel person to a good one.

Other details I think I remember:

The Darkangel Trilogy by Meredith Ann Pierce. From Wikipedia:

In the end, Aeriel finds that she cannot kill her darkangel because he is still beautiful, and because there is a trace of good in him. She gives up her life to save him, cutting out her heart to replace his, which the White Witch covered in lead. The duarough, Talb, who witnesses this, takes the former vampyre's (now reverted to his human name of Irrylath) heart, removes the lead, and places it into Aeriel, saving her life and making them one.

Also:

It is set in the distant future, when the Moon has long since been terraformed into a lush paradise complete with its own animals, plants, and races.

From a Thriftbooks review:

Meredith Ann Pierce's "The Darkangel" is the first book in a stylized epic trilogy set in a world that is so dry and thin-aired it can barely support its population - although it manages to do so through magic and machinery. The world has been cursed by the White Witch and her seven unnatural sons, the darkangel vampyres. Aeriel is a slave, a foreigner in Avarra, the country of her mistress, Eoduin. [...] All their souls hang around their husband's neck in vials of lead, to be given to the Water Witch, a lorelei, after he collects just one more. In only twelve months, the last bride will be taken and he will become the true seventh vampyre son. [...] He himself has been cursed by the Water Witch to live with a heart of lead.