66 barracuda driveshaft shortening

So I was told that there is a drive shaft that cannot be shortened? Tube in tube? How would I identify that?
Two shops mentioned that I might have to have a new one made up after telling them the make and model. I don't find that issue mentioned in the thread searches.

I have the 489 -8.75 case set up in the 66 Barracuda behind a 904.