Level chassis in both dimensions. Use a tough thread, running front to back through a line marked at the center of the bottom frame members as the center reference line. Use another piece of thread to mark the centerline between the two lower ball joints (this defines your wheel base). The thread can be held in place on your build board with push pins.
The lower arms locate the suspension. Fit brackets to lower a-arms. Set the rear bracket on the chassis, use spacers to set the arms level in both dimensions, then offer up the front bracket to the L assembly. Adjust the bracket locations until the ball joint will be in the correct place with the arm centerlines aligned and square to the chassis centerline. Ensure both ball joint centers are square to and an equal distance from the center line. Tack lower brackets into position.
Now, fit brackets to the upper arms. Find center of upper arms threaded bush which will give the correct steering castor. Lightly clamp front brackets to the L members at the correct height. Now the trick is to get bush centerline in the correct position, with the arms level in both dimensions at the correct height. The other variable is the location of FU1 & FU2. The goal is to position the upper arms and FU1/2 at the same time, tacking all into place when properly located. This is a tedious exersize, took me the better part of a day. One more tip to make your day interesting - put a long bolt through the rear bottom brackets.Make sure you position the bottom of FU to allow these bolts to be inserted completely. Trust me, this will make your life much easier...
I would wait until the chassis is pretty much complete and the hubs are in place before finish welding the brackets and FU1/2. Do one last check of the alignment before welding everything up, this will allow you to make adjustments if the chassis has shifted at all during final welding, or if your measurements were inaccurate during intial setup (it's quite difficult to measure the bare a-arms accurately).