Arm Rest Base

[olddman]

Doing some work on my 1965 Dart GT and I am going to replace the front and rear arm rest bases, but not sure how to measure them. Have found them to be offered in different lengths and want to be sure I get the right ones. Can someone tell me the right way to measure them?
Thanks!

 

[SSVDP]

Hope these pics help.
These are the three arm rest bases and the two sizes of arm rest pads that fit early A-bodies. You probably need the middle base which is approximately 11.5 inches long with a pad 10.25 inches long.