NEED HELP!! 1973 Dually Brake Problems

[Crcrft78]

Does anyone have a part # for the rear brake wheel cylinders? I have tried 3 sets and the first two where 1/4 inch too wide and wouldn't fit without me grinding on the bracket attached to the backing plate. The 3rd set was the correct width but the threads for the brake line were 7/16-24 and my brake line has a 3/8-24 fitting. The auto parts stores all list wheel cylinders for every year except 1973. It seems like that is a problem year for parts. Maybe because of the model change I don't know. Any help would be appreciated.