42RH/A500 vs. 46RH/A518

I need to replace the blown trans in my 72 Coronet wagon. It's just a cruiser so mileage (haha) is a primary factor. Its a plain jane 318 that will go to a 4-bbl and a mild build. My plan was to put a 42RH in it but they are proving scarce. I can get a 46RH pretty easy.

Question is, what is the efficiency difference between the 42 and 46 for parasitic loss?