Slant 6 and 318 trans options

I would start by wandering over to the MyMopar site and download a service manual for your year (you may have to satisfy with Dodge/ vs Plymouth, as well as get a year or two older/ newer and download those as well

The problem is that "actual numbers" are not always used. You can put EITHER a 904 or 727 auto behind a 318. A 727 for a slant is rare and hard to find. The are in fact different main housings between slant/ small block, and big block AND be aware there are some oddball Chrysler autos that THE CASES WERE CAST to fit AMC or other engines, and therefore will not fit any Chrysler

The 904 is same length as an installed 3/4 speed stick, while the 727 is longer, so takes a different driveshaft

The same transmission can "sorta" be used for either floor or "tree" (column) shift but I don't think the older box was used in floor shift. The older box is easily identified by a top cover, the newer "all syncro" (and stronger) 3 speed has a side cover more like a 4-speed

The lighter duty, older design, and not synchronized into 1st gear box is an A903
The heavier duty, newer design, "all synchro" box is an A230