I have an SM319 (55 Muncie in front of a 58 or so BW overdrive), but the 3-speed guts are the same as the sm318. I don't know if yours is the same.

Mine had 14 fat roller bearings and 24 thin ones. The fat ones go in first (into the input shaft).

Take a look at this thread, post number 993870.

Keep looking!

Best,

Jim

1957 Chevrolet 3100 Stepside
235, 3-speed w/OD
In the Stovebolt Gallery