2by4by.120. You can taper the rails like a stock frame by wedge cutting each side, then squeezing them together to reweld. Make sure to bevel the edges on any welds, and if your welder isn't very heavy, either get one that is, or turn it up all the way and make multiple passes. One pass down the middle of the joint, one on either side of that weld, and then finish up by putting a "wash" over it...weave a bead back and forth accross the three welds, extending about 1/4" on either side of the welds. This is the way to make your joint its strongest. Incidentially, if you put a bead across the width of non boxed material, and the material is subjected to stresses, it will almost always break on the weld because it is more brittle there than any other place.