From my experiences with the '66 Newport, you will need an idler arm far sooner than a pitman arm. Might even hear some "clunks" on uneven pavement and a decrease in precision in the corners. I suspected the idler arm was worn when I couldn't follow BMWs through an off-ramp on the freeway (circa 1974). But when the new Moog item was installed, I could really intimidate them.
Other than the noted coupler, with the engine off, turn the steering wheel and see if there is in 'n out movement on the input side of the steering gear. There is an adjustment there, too, although few seem to known about that one. The rest of the steering components can be "in spec", but if that gear input adjustment is loose, the whole system will feel loose, by observation.
IF you are seeking the car to feel like "a newer" car, you can chunk parts at it all week long and not get there. Unless you upgrade to the Borgensen gear and such. But the OEM design is a good one, anyway.
Might check out some of the Chrysler MasterTech videos on suspension and steering at
www.MyMopar.com for good measure.
Be SURE that what you might desire to replace really needs replacing FIRST. The OEM items are probably better than many of the new replacement items on the market today.
Just some thoughts and experiences,
CBODY67