Torn, if AFS supports lon/lat import of soil type data, wouldn't he be best served to download lon/lat (Geographic) data, and let AFS do any required reprojection on the fly?

FWIW:  NAD83 and WGS84 are effectively the same for vector data.   Differences are well less than the accuracy of the data collection.   However, for imagery, many packages require an exact datum match based on datum codes, even though two different defnitions are mathematically the same under the hood.