My friend was a master machinist welder.He made many drive shafts for cars and semis.Never thought about trying to lenghten one.Made one for my Jeep from oil well pipe.He would cut off the ends in lathe then weld in new tube.Made one for semi wrecker I knew of for sure.Guy snapped like 3 factory ones.His ran for 5 years and was still good when he sold the truck