Where did you buy it from? If they promised bolt in. That is not Dana or Strange's fault. It is whoever cut the tube length and welded on the control arm brackets. Good choice on rear btw. They are not as popular but they are way stronger than a 9" Ford. Mopar musclecars run them in heavy cars into the 9's with stock internals. Any 9" at that level has zero parts from Ford inside.
NorthernRegal mentioned size. This is a huge factor but drag racers do not mention room for diff size because they run spools which require no space. In a 9", there is no space. A 9" Trac Loc is a puny little item because of the pocket bearing. A Dana60 or S60 has a huge differential. I know you bought a helical gear but the option is Power Lock is there and that sh*t is strong. Also the smartest differential ever made. There are ramps on the pin for the 2 spider gears. When the ramps see torque, they crush the clutches which locks them and acts like a spool. Every other clutch type diff relies on spring pressure to push on the clutches so the spring you chose is how much it pushes on clutches whether you are on jack stacks, rain, drag racing or street.
I will probably go with a GM 12 bolt. Not because it's better than a S60, only because it's GM. The 9" is an option for spool, but if you want a differential 9" is out due to it's tiny trac lock. As for S60, congrats on the strongest diff known to man.