It is my understanding that it was on the Challenger R/T, T/A and Rallye optioned cars EXCEPT the very early 70 R/T's didn't get them automatically.
The Cuda & AAR got them but I am not sure if they were on the very early 70 Cuda's.

There was probably other options that I don't know off the bat but somebody will come along and correct me if I am wrong..

I believe there was a 340 Barracuda that technically wasn't a Cuda but trimmed out the same as a Cuda. Correct me if I am wrong.

As for cost they were pretty common. The last time I bought one it was less than $100 for the whole setup but that was over 10 years ago.