Make sure the frame is setting level and secured at the point where you will cut it, don't want one or the other rail moving on you!!!! Measure, Measure, Measure, think awhile, look things over again, Measure, Measure, cut..... Then when you get the sub tacked in place, Measure some more, weld, fishplate, and gusset the joints back together.... Not a real tough job, patience and careful cutting and fitting is required. When it's all done, go out and enjoy your '54 with the good handling and braking!!!!!