[mr48chev]

When I subframed my 48 I ran a big ugly master cylinder and booster off a donor car that we scrapped out and that one would put you though the windshield with the same brake rotor and probably rear drum setup you have but it was horrendously ugly under the hood. That it dominated the whole under hood area and hid the engine from the drivers side is the main reason I don't like firewall mount boosters in old trucks even thought they usually work fantastic except when you get the wrong combination.

One thing on old trucks brakes or handling is your perception of what good brakes are from driving your daily driver. If you are driving a late model somewhat performance car or even half ton truck with 4 wheel disk brakes you may be expecting the 56 to stop the same way that one does and while it is actually stopping exactly as it should it isn't stopping like the daily. If you daily is a mid 70's C_0 the perception is going to be different.
Have you got a buddy with a similar truck with close to the same setup on it that you can have drive it and get an alternate opinion?

The other thing is brake pads and brake lining, Some just flat doesn't stop as good as others but may be quiet and not make as much brake dust. I've got a set of pads on one of my OT cars that I am not real happy with as they don't stop as good as the worn out pads I took off. I should have spent a few bucks more for a better performing pad.