42RH/A500 vs. 46RH/A518

[halfafish]

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?

 

[Kern Dog]

Traditionally, the 727 was likely to slow a car down by .15 in the quarter mile when compared to a 904.
The 518 is 727 based so the math probably still applies. For the extra girth you do get increased durability. The wagon had a 904 last this long. The A-500 series is a bit more robust than most 904s were plus it has a deeper 1st and 2nd gear when compared to a 518.
2.74 vs 2.45 will make a difference in acceleration from a stop.