Every master cylinder I've seen has the retainer groove. I have a power-brake cylinder on my manual-brake '69 Valiant and the groove is there.
I don't know what year your car is, but in the 1969 parts catalog there's no distinction between manual and power brakes on A-bodies. You should be able to get away with a manual pushrod, the special O-ring bolt that secures it to the pedal, and a new master cylinder. The last power cylinder I ordered came with the rubber retaining ring, even though it wasn't needed.
Personal opinion: I prefer manual brakes to power, particularly with discs. Four of my six cars have manual brakes and I'm strongly considering removing the boosters on the remaining two.
