One place to start would be to find all of your manufacturing tolerances, all of you stock section tolerances, then compute the volume for MMC and LMC. Then find out the tolerance on density, and compute you max weight and min weight. You'll also need to get all of your bolts and weld beads with such a tight tolerance. However, that is a lot of work, and will probably not yield good results.
I might just add lightening holes or steel weights after assembly, assuming you can weigh it. Then you can tweak the CG at the same time.
I would first ensure that the values you are using for your structural shapes are the design dimensions and not the rounded detail dimensions. This can cause all sorts of headaches for you. Check out the AISC website where you can download some tools (xls) with the current acceptable values for steel shapes.