Mark's response brings out some good points.

"In both cases I just used combinations of parts that were similar or identical to those in later vehicles that originally had the dual system."
"The proportioning valve needs to either be adjustable or one that is from a similar weight vehicle with a similar brake system."

https://lugnutz65chevystepside.weeb...aster-cylinder-brakes-1963-1966-c10.html

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."