I'll just say... be careful....
That's another one that someone REALLY needs to look it over because there's some bad info in it. Been up at moparts for a few years.
So theoretically, if I have 1975 slant 6 duster, 904 w/ a 7 1/4, and I have a 1969 barracuda 340 727 w/ a 8 3/4, and I want the 69 drive train in the 75. I can swap the drive shaft, and rear into the duster first. Allowing me to drive that slant 6 / 904 combo, until I'm ready to do the engine, and trans, and all the parts will work properly.