Pst suspension parts exchange needed?

So I swapped my 73 k member, s6 w disc brakes for a 71 k, v8 w drum.
Only thing I see needing changed is the idler arm. The rest of parts arent bolted to k member.
Contacted Pst and was informed now my ball joints both upper and lower will not work now.

Can anyone tell me why?
From what I see, I don't see any issues...

Keep in mind, all I swapped was the k. Car is still disc.
If I went to drum, I could see needing ball joints.