this answer is a 'for what its worth'. I asked this question recently of our resident expert on Dodge's for my own restoration. he said the 4 blade fan was stock, but the 6 blade fan cools better. I went with the 6 blade fan. my 41 parts truck had the 4 blade fan, and the 41 WC6 i'm restoring had the 6 blade fan.
42 GPW; 41WC6
Korean War: GEN Paik Sun Yup; "I shall be at the front. If I turn back, shoot me".