Heres what i do personally.
In my opinion, you need to use a frame table of sorts. Lots of people have built them on the ground, and they have been successfull, but having the frame on a table makes it that much easier to work with .
I layed my frame on the table i made. Then welded some tabs to fix the front end and the rest of the frame directly in place. I then welded nuts to the table directly under each bodymount, and ran a piece of threaded rod through the body mount location, then got a piece of steel like a washer to weld on the rod to mark the height of the stock bodymounts. After i marked all the heights, i cut the frame off ware i wanted to start the new frame. Layed my 2x3 out, and then threaded the bodymount markers back into their spots and put a spacer on them to drop the height of each location the amount i want to bodydrop the truck, then my new mount would but right up to that and go from thare.
idk, if thats confusing or not. check this profile of mine for more pics
Http://www.streetsourcemag.com/Profile.aspx?ProfileID=42921
heres some pics: