Here's some pictures of the PS3000 that I modded so that I could install a double-sided floppy drive. I did not have to modify the power connector. It fit the double-sided drive without any modification. The data connector was the only part that needed to be modified. For the data connector, I had to remove the original mylar connector on the board and solder in a 34 pin ribbon cable with the appropriate connector attached. I figured out the wiring for the 34 pin connector by comparing the external drive connector with the pads on the board.
I also had to remove the bottom shielding of the double-sided drive so that it would fit in the monitor. The two mounting screw holes for the single-sided drive are in the same location as the double-sided drive. I used the original screws for the single-sided drive to bolt in the double-sided drive.
I hope this helps. Let me know if you have any further questions.
Almost forgot about the Head Select and the Device Select pins.
-I cannot recall whether the "Head Select" pin was used for the original single-sided floppy in the PS3000. It definitely has to be wired for a double-sided drive.
-I was using this PS3000 with a Mega ST, so I had to wire the 34 pin connector so that the floppy drive in the monitor was "Drive Select 1" instead of "Drive Select 0". If I did not do this then both the PS3000 drive and the internal drive in the Mega ST would both respond as drive A. The PS3000 comes up as drive B when it is wired to "Drive Select 1". If you have an original 520ST with no built-in drive then wire the PS3000 drive to be on "Drive Select 0". That is unless you plan to run with two drives on the 520ST. If you plan to run two drives then wire the PS3000 drive to be "Drive Select 1".