You can’t use your existing 7 1/4” lines, the splitter block is a different configuration. You also can’t use your existing 9” brakes, as the A body small bolt 8 3/4” rear has a unique offset axle flange that will only work with the 1 3/4” with 10” brake setup! The backing plates from the 7 1/4” are also different and only have 4 bolt holes instead of the 5 that the 8 3/4” has.