If it was a SBP, 4 wheel drum car that might be possible. 4 wheel drums were still standard for a 73+ slant 6 car.
But plenty of /6 cars ended up with disk brakes on the 73+ cars, and if a 73+ car had front disks it was BBP regardless of its engine. And all the BBP cars had 10x2.5” rear drums. Doesn’t matter if the service manual says it could have come with 9” drums or not, if it’s BBP it doesn’t have 9” drums.
