Slant 6 and 318 trans options

The A903 is most likely what you have because of the slant and three on the tree Duster.
They also made the A745 (looks the same as a A903) that was used for Taxi and Police 6 cylinder cars.
You can use the A903 behind a 318 with the right Flywheel,Clutch/Pressure Plate and Bell Housing.
You also can retain the column shift. You may want to look at the data tag/fender tag on the left front inner fender.
The lowest row of numbers on the tag,second from the left will be a 3 digit number.Most likely starting with a letter.
THIS IS UNLESS YOUR CAR CAME FROM THE LYNCH ROAD PLANT.
If so the trans code will be the third set of digits. I do not know what the code is for a A903.Going to My Mopar as
67Dart273 stated and doing some research is what you need to do.
Fun huh ? Welcome to the Mopar world.