Somewhere a while back I read that the difference between NAD 27 and NAD 83 was on the order of a few feet at most and thus I had ignored it. Turns out I was very misinformed. NOAA has a site where you can calculate the difference between NAD 27 and NAD 83 which I tried today and my coordinate varied by more than 50 meters. That's pretty significant. So, lesson learned.