Mark,
Although Clausing at last check still maintained a stock of the 6" and 10/12" halfnuts and some of the lead screws, it is highly unlikely that they are doing so for the double lead 3/8". So if his half nuts are badly worn, he probably will not be able to get new ones to replace them and since the larger half nuts will not fit the 3/8" lead screw, he will have to replace the screw, right bearing and left bearing and maybe some of the gear train as well. And I have no idea about the related parts in the apron. Had I been running the Atlas Engineering Department in 1938, I wouldn't have let them use anything except the standard 6" parts to begin with.