Look for ATX conversion guides on xlr8yourmac.com, they should have the pinouts.
In regards to the PSU, the minitower and desktop used the same (ATX compatible) while the AIO used a different one.
To find jumper J28 look at the board from the side of the rear ports. See the PERCH/Personality slot(with the PCI slots to the right of it)? Well, look at the rightmost PCI slot(closest to the edge of the board) and look at the side of the PCI slot that is closest to you. There should be a hole and the CUDA reset switch near it. It is a relatively small jumper IIRC but it is marked (on my board anyway) so should not be hard to find.
In regards to the PSU, the minitower and desktop used the same (ATX compatible) while the AIO used a different one.
To find jumper J28 look at the board from the side of the rear ports. See the PERCH/Personality slot(with the PCI slots to the right of it)? Well, look at the rightmost PCI slot(closest to the edge of the board) and look at the side of the PCI slot that is closest to you. There should be a hole and the CUDA reset switch near it. It is a relatively small jumper IIRC but it is marked (on my board anyway) so should not be hard to find.