Yes, you can just swap the mainshaft and tail housing, everything else is the same. It has to be the 66-67 slant 6 or 273 mainshaft and tail housing together for the 26 spline output or 68-75 standard mainshaft and 68 up A/F-Body tail shaft for the 30 spline output. The small O/D tail housing only works with the small 26 spline O/D main shaft. The O/D mainshafts and gearsets are different. Early standard gears are nice strong gears and will fit any standard mainshaft, big or small output splines.
