Go to open tracker.com
They have limited stock of moog lower ball joints for 65-66 mustangs.

NAPA lifetime are indeed good balljoints.  The stock lowers on my F250 were shot at about 25k miles.  I replaced them with NAPA and they lasted to 120k, I just put another set in at 210k, and the best part is I paid for only one set.  That truck basically torture tests balljoints.  IMO it really should have kingpins because its just too heavy, so getting 90k from balljoints in it is pretty good.  In a car application I doubt they would ever wear out if kept greased. 

I went with the NAPA Premium Grade arms with Ball Joints and Bushings, with AAA discount they were $65 each and free next day shipping.

MOOG seems to just offer the R-series parts for the stang winch is their economy brand, 3 year warranty vs lifetime, and I thats what i have on it now.

Fingers crossed,

Yea, these are defiantly worn out, I can have someone move the bottom of the tire in and out and watch the movement, and I think the rubber boots were split for years by all the grease that I cleaned up. What ever brand these are (came on the arm in a kit) they don't look like much. 

MS wrote:

TKO, I question whether truck ball joints of a particular brand are even relatable to the early Mustang design as far as quality. Might be, but still questionable comparison in my mind. I do agree NAPA probably better than Other inferior stuff. But, most stuff I have seen there is simply some other brand in a NAPA box.

Ok, back to original question. Do the ball joints have that much slop (wear) or are you simply depressing the internal spring that holds the ball tight in the socket? Remember the old trick mechanics used to sell you new ball joints? They stuck huge channel-locks on the joint and compressed the spring, showing the customer how much vertical movement they had. Remember, the lower joints actually do not support the weight of the car. That is the upper joint’s job.

Ten years of intermittent use by a caring owner that keeps them greased is unlikely to wear out a set of lower joints. Unless they were those PRO brand that unfortunately hit the market when reproduction parts started hitting the market. Those were unsuitable for use the day they were installed.

The top end lifetime stuff from NAPA is built under contract by DANA regardless of application.  The lower end stuff is Master Ride built under contract by McQuay-Norris.  I only use/recommend the lifetime stuff.