What did the A518 come in?

Just curious where you would find an A518 transmission?

Also, I have an overdrive transmission mated to a 318 from a mid 80's Chrysler 5th Ave, I am wondering if its a 904 with overdrive? Or if it is something else, how to determine what it is? I know the difference between a 904 and 727, but not up to date on my other transmissions.

I'm asking because the 904 in my 65 Valiant leaks where the cables enter it, and being its a daily driver with a 273 2 bbl, I'd like to get a bit better fuel mileage on the highway. I'm considering swapping in a transmission with overdrive.