sounds like a good plan. Might also want to figure a way to build a fixture that will hold the rear section securely while you do the cutting and fitting. When welding it together, don't try to weld it all at once as it can distort the rails and mess up the squareness of the frame. Some strong, well placed tacks and short welds on the pieces with sufficient cool time will prevent this.... Measure, Measure, Measure....Patience, Patience, Patience.