The 5 number 14 x 3/4 self drilling screws I used on the corner plates are STRONG. I like them because I avoided drilling holes through the door.
I wish it was easier to remove springs. I'd remove the one in the center (giving me 4). If I can get the end of the door on the ground, without too much pressure on the hinges from the springs, and still be able to raise and lower the door by myself, I would be very happy.
A current unknown is how much the springs will "give up" with use. Here's how my door hangs with 5 new springs...