Rav Moshe did not allow receiving heart transplates or even open heart surgery, he considered doctors who performed the transplants to be committing murder.
It is also possible that he did not allow it because the mortality rates from such surgeries was very high and the life expectancy of the patient was rather low.
Those conditions, thank God, no longer exist. Do you know of any *contemporary* poskim who prohibit heart surgery across-the-board?
The Wolf