I will add that if you are planning on installing an Export Brace, temporarily install it prior to welding in the subframe connectors. I didn't and had a harder time with their installation because the front frame could no longer "relax."
You need to have weight on the suspension when you weld in the subs so as not to cause a bind in the chassis when you lower the car back on to the wheels. This can be done with stands under the rearend but you need to have the front supported under the front suspension so it is compressed. Also remember to have all the doors, hatch/trunk or convertible top closed during this as well.
i would want the engine in the car. if possible use a drive on lift, & do like bad gt said= doors hatch/trunk , windows, hood all shut or closed.check doors to see if they open/shut normal after job completed. good luck.