I just did this conversion the other day. Drill center of hole 13 inches from the front of the open fridge. I didn't encounter any cooling lines on the top of the fridge. Definitely want tower far enough back to avoid the bolts hitting the top of the kegs.
Just built one using this fridge, and like OP I drilled the hole centered 13 inches from the front of the fridge (while the door is open). For me, bending the freezer shelf down to place it along the back wall was the hardest part. There didn't appear to be any cooling lines on the top of the...