Great post with lots of amazing resources for finding photos. Most of the photos you posted I hadn’t seen before. And you are correct about this photo:
It was jammed in an envelope with the mystery photo so I figured it was the same spot when I noticed the marble bedrock. However, the gas station in the background was at the corner of W. 230th and Broadway so it has to be the foundation of the post office. Good sleuthing. The NYPL has a photo of that gas station here and it is labeled as W. 230th and Broadway.
Tieck’s red book has a good writeup on the construction of the post office including the human remains (possibly from the Revolution) that were discovered during its construction.