'73 up brake distribution block/proportioning valve question

The 1976 model is different from earlier models.
The master cylinder, calipers and proportioning valve were changed in 76. The caliper pistons increased in size which probably required a new master cylinder and proportioning valve.
When doing a drum to disc conversion it would be much easier to find the above mentioned parts from a 73-75 abody. The proportioning valve in 76 was a one year only part.