Any modifications to the mounting points on the female end of the 3M riser?
None needed. You would just need to make sure that your graphics card has a backplate because unlike the LiHeat riser, the 3M riser's exterior is conductive. If it does not have a backplate, you would need to add some insulation between the riser and graphics card PCB.
#6-32 type ??
They are 5.8 mm in diameter, so they work with self tapping screws for computer fans and 6-32 radiator screws. Heads of other 6-32 screws may be too small and require washers.