Not a problem with 64-66, the rod was not threaded there. The 67 was, that's one year only in the Mustang. The 68-69 rod was a bit slim, the 69 BOSS 302 had thicker rods. In 70-73, all Mustangs got the bigger BOSS size.

Even on the big BOSS rods, the easy way to avoid this is to not use urethane strut bushings. They are OK in some places, I like them in sway bar end links, because they don't add stress and do deflect less than neoprene. OK on sway bars, except after a while they start to squeak. Silicone grease or teflon tape can help there. Other places tend to make the ride harsh without gaining you much, on a driver.

 

I've been running urethane strut rod bushings for years and have 0 issues with them. If it really was that much of a problem no one would make them, there'd be too much liability involved for the company. There's dozens of reasons a structural component can fail, including but not limited to being over tightened, corrosion, porosity during manufacturing etc. Even with the urethane bushings my strut rods still have play and don't bend, and steel bends quite a bit before it breaks.