HI..I ran into the same problem when I was trying to research my grandfather and great uncle who were at the GPO also in the 1870s. If my memory serves me right the 1880 census listed all of the children in the orphanage at that time and it gave me a few leads. I lucked out that I knew my great uncle was in the Spanish American War..long story but I got tons of info from the gov about a widow's pension request. I had looked for 50 years for info about them and it all popped in a year or so. I used the city directory to get some info about where their family lived. I must admit I fell into some of the info out of plain LUCK. I've found that if you use every tiny bit of info that you have you may be surprise. Good Luck..Oh yes I've been to the GPO address on State St in NO and have pics of it before it was torn down! I talk too musch! Yell if I can help.