It doesn't matter where the brick is, as long as you have enough room to clear the diff housing with the string. If you held the string with the bottom of the tire, you wouldn't be able to swing the string up straight across the car because of the housing in the way, that's all.

Yeah, you definitely want the car at ride height so that you get the proper compression and extension on the shocks when it's finally goin' down the road. What you can do is approximate the position of the upper mount and make several different mounting positions so that you can use the one that is closest to being right when you have the car together.