OpenGL uses column-major order and an appendix in the OpenGL "Red Book" (http://www.glprogramming.com/red/appendi...) shows the translation values in the right-most column.  Since the GIFTI spec indicates that the transformation matrix data is in row-major order, the translation values must be in the bottom row and hence values (m13, m14, m15).

I don't want to answer the pre- or post-multiply question.  The best way is to create some data and test it.