Steve is correct- drill the center of each ball joint. I've also not seen a worn upper assembly. Zerks are usually 1/4-28 SAE thread but Dorman has some others. On the upper rear, you will need a 1/2" extension so the zerk sticks up enough to get the grease gun on. And for convenience if you don't have a hoist, you can use right angle zerks in the lowers so you don't need to raise the car to the moon for greasegun access. When tapping threads you'll only get about 1-1/2 threads in the sheet metal housing, but thats enough as there's no pressure involved. Do NOT overtorque or you can easily strip the thread(s)! Note also the tie rod ends can also be done.