UPDATE (Aug 2009): With some prompting Trimble has added AREQ back in the list of SOPAC reference stations for differential correction. If Arequipa Laser Station (AREQ), Peru doesn't appear in Pathfinder Office Differential Correction provider list, click the "Update List" button.
ArcGIS offers a number of geographic transformations between coordinates in the older Prov South American Datum of 1956 (PSAD56) and the modern WGS 1984 datum.
ESRI Press has published a book on this topic titled "Lining Up Data", 2010.
The PSAD1956 datum is supported in ArcGIS. The coordinate system settings are found under Coordinate System menus under UTM (Universal Transverse Mercator)
Summary: This technique allows a user to bring scanned maps and other rasters from a variety of projections and coordinate systems into a common reference system. The idea is that a vector file can be transformed more easily than a raster file, so a vector border of a raster map is transformed (in lieu of the raster itself), and then the raster is re-georeferenced to the new vector position.
Maps published in a number of Andean countries in South America (i.e., Ecuador, Peru, Bolivia) are still often based on the Provisional South American Datum of 1956, La Canoa (Venezuela), abbreviated to “PSAD56”.