Hy Maxine, welcome, I checked my United brake catalogue, for your truck the flexible lines that are shown as replacements do not use an outer threaded attachment at one end. The lines they specify use a horseshoe clip to retain the lines to frame brackets. The part numbers I come up with are UP13841 for the fronts and UP4497 for the rear, those are Napa part numbers. If you want to check RockAuto the Raybestos part numbers would be BH13841 for the fronts and BH4497 for the rear. Incidentally all the lines I saw in my catalogue that used a 5/8" thread had an 18 t.p.i. pitch, hope that helps.