You are correct. I am talking about convictions and sentencing after the 1972 decision. If you look at the current Florida Death Row roster, there are condemned murderers sitting on Death Row now who have been there for over 35 years, some who did their killing in the 60’s but were convicted and sentenced years later.
The Constitution explicitly forbids ex-post facto laws. Any attempt to execute someone for a crime commited in the 1960’s using a law passed in 1972 would seem to me to run afoul of the Constitution. You have to be tried under the laws in effect at the time of the crime.