OK, then see the chart at
http://www.hamtramck-historical.com/images/TSBs/1970/D70-3-2 page4.jpg
It is for 1970 Challengers & Darts, but the 3 digit numbers are the same for the Plymouths.
The one with 10" brakes & the 3.55 is what you want, but compare your center section type & u-joint yoke size too just for fun.