60-66 interchange, if the bolt on K frame is rusty then your car has big rust problems.
the frame rail replacement can be sectioned or the whole piece replaced back the the transmission crossmember.
have your body man check them out carefully-- there is a member on here that sells very nice replacement pieces from time to time & he completely removes the part at the spot welds, I cannot recall his name at the moment.
Post up some pictures & have fun, Lawrence