[dsraven]

using the plumb bob idea works but could be off a little depending on if the frame was set level at the time.
when doing the cross member it is important to have the frame set up on stands that will hold the frame true no matter what kind of work you are doing. sometimes installing a cross member can mean some muscle work gets done and things can move a little when that happens. I usually recheck the frame several times as I go through the procedure just to be sure. I will say again that a digital level is the way to go here. I used a bubble level and it looked fine, or so I thought when setting up, but when I put the digital on top it was off by several degrees. I realized that it can look different, as far as what you think, when you look at the bubble and the lines around it from different angles. the digital level can't lie that way because it is an LED readout screen.
I used a set of jack stands with the sliding bar removed, a plate tacked on the top with a hole in it, like a washer, and then used 3/4" threaded rod sitting on that with a couple of nuts (one to set the height and the other as a lock nut )and the other end of the rod attached to the frame. the top end of the rod had a 3/8 bolt welded to it which went through an existing hole in the frame with a nut on the other side that got snugged up once the frame was where it needed to be. level it side to side and then set the rake angle you want for the fore to aft angle. grab a sharpie and write the angle on the frame so you don't forget over time as these things can take your memory for a ride with all the numbers, lol. try to place the stands where they won't be in the way for removal or installation of the old and new. when levelling the new cross member be sure to take the levelling across the lower control arm pivot bolts and not off the top of the bent steel part as it can be off in comparison simply due to the way it was all made. I will see if I can find a few pics of some I have done and post them for you to check out.