1967 Dart stiff steering after replacing Idler Arm

Remove the frame end and see how tight it is in the centerlink; that is to say, how difficult is it to rotate. If that end is free, then check the other end,installed, and the center link split off.
I swear some of the aftermarket stuff is pure junk. Moog and TRW used to be the suppliers of choice, but I heard they are now also manufacturing offshore.
I noticed the change in around 2008 when I was still an alignment tech.
Good luck.