Are you planning to stay with the drum brakes all around, or is an upgrade to disc brakes in the plan? If the drum brakes are going to be retained, you'll need a MC with the same size reservoir for front and rear. The disc brake type MC has a much larger reservoir for the front brakes, and you'll run out of reservoir volume before the rear cylinders are displaced far enough to apply the brakes. A master cylinder for a big Buick, Olds, or Cadillac with drum brakes all around, maybe late-60's or so, will have large reservoirs for both front and rear, and a big enough bore size to work with the large wheel cylinders of your truck. It might also be wise to install a proportioning valve to control rear brake application when you're running unloaded, to prevent rear wheel lockup.
Jerry
