My A66 Challenger 340 convertible has them on both front and rear.

70 six pak challenger.

my lowly 318 71 barracuda has both built june 14 had 7.25 rear axle

Most 71s have the fronts. Having the rears would be odd for that application unless it was a convertible

I agree with Morty426 - I've never seen the rears on a 70 or 71 with that engine application.

My 71 383 4 speed, A31 axle package has front, my 70 440 auto 355 8 3/4 has front and rear, and my 70 340 auto air has front and rear, also has rear sway bar (torque boxes seem original, rear sway cant tell, LA built no broadcast sheet). I had a 71 GC 318 auto air no torque boxes.

forgot to mention my 71 318 7 1/4 is a convertible

they are technically frame connectors.
they connect the front and rear framerails through the rocker panel, which btw convertibles had heavy 1/4" angle iron spot welded to the inside of the rocker panels for rigidity.

i had a 1970 318 i forgot if it were 3 or 4-speed on the floor, but it had front and rear tork boxes, i would assume a factory mistake.