For 1973, the residual pressure valve was deleted from the master cylinders across the (Chrysler) board, because drum brake wheel cylinder seals had been improved enough to render the RP valve unnecessary. It is just about 100 per cent certain any pre-1973 car still on the road already has post-1973 wheel cylinders with the improved seals, so you probably needn't worry about it. On the other hand, if you'll sleep better at night, you could put in an RP valve, either in the master cylinder outlet or in line, and it wouldn't hurt anything.