I had so many failures of the junk MG996R servos I got that I decided to use the DS3218 servos for the linear motion as well.
It is a tight fit where the little groove gets filled with the servos angular support piece but squeezed in OK when tightening the screws. The other change is that the 3218s shaft is about 2mm longer when mounted. The pinion fits but just barely. I chose to make the hole for mounting the horn 2mm deeper and reprinted the pinions.
You'll also need the horns from some MG996Rs. I used the ones from the junk servos I returned to Amazon.
The servos from Amazon were so bad that I was able to cause failures by simply exercising them using the Pololu control panel under no load. When they warm up, they die. The failing ones say TIANKONGRC as the brand. Genuine Tower Pro servos may fare better.