We went from metal to plastic on the spacer, they are interchangeable.
The piston retainer is the part with the clutch splines, also called a rear clutch piston support. I don't recall a change to it in 74.
The clutch retainer is what fits around the piston support and that is what had the groove location change made to it in 74.
As the bulletin states it is interchangeable. If you can - you might try to add another snap ring to the one just up from the spacer, it makes the retainer tight to the support if you find yours is sloppy.

As for the retainer - a retainer out of a 46/47/48 will fit the 727.