Boy, tough question.
Pearson was known to use and re-use whatever they had on hand. Thus, different models built around the same time often shared hardware in common (cabinet latches, cleats, running rigging, standing rigging, etc) , and same models built years apart could have significant differences in the same hw. I don't know if they ever used more than one spar type on the 323, which would completely change the masthead fitting.
On the other hand, our two boats were built pretty close together. Mine is hull #42, built in Dec of 1976. Since the 1977 model year began in August '76, this means they built 42 hulls in only 5 months! Since yours is hull #74, there's only 32 hulls between us, so I'm guessing yours was built around April or May of '77, still in the 1977 model year.
This would suggest that we have the same spar cross section, and the same masthead fitting. I've attached masthead photos for comparison.
If they're the same, I would expect the sheaves to fit. Even if the mastheads were hand-built, I'm sure the sheaves would have been an off-the-shelf item for them, and the mastheads would be built to fit them.
When I redid our mast, I was concerned with the close fitting of the sheaves in the slot, and worried that three layers of Awlgrip (plus primer) would build up enough to cause them to bind, but they fit perfectly.
The difference was night & day. The old aluminum sheaves were seized on their pins, and rubbed the sides of the slot. This, combined with stiff wire halyards, meant that both sails had to be pulled down, and it took some muscle.
The new, slippery Delryn sheaves, with fresh bronze bushings and stepped sides, combined with new VPC halyards, allowed either sail to fall to the deck when the halyard was released.
In the end, its your call. My OCD would probably make me wait until I had the old sheaves in-hand to measure, just to be sure, but that's just me. I tend to overthink everything.
EDIT:
In order to get the seized pins out, I had to use a pin punch and hammer. The spinnaker beak blocks thru-access to the forward pin, and I had to drill an access hole in the beak (visible in the third photo) in order to drive the pin out.