Yes PIN 7 is a ground of the APW7067N chip. Any ground will do fine, however if you are not so good at soldering, choose a larger more easily accessible ground like one of the grounds of the 6 pin PCI-E power connector.

You should also buy a DMM, even a cheap one, there is no way you should do the mods without it. You need to find alternative grounds, test your soldering work before you power on the card, and use it to measure voltages after you have done the mods.