Trucks like the Chevy C60's did indeed use hydravac's, as did the WWII CCKW. It would be easy to take a plain old 2 1/2 ton Bendix Hydravac and hook it up to a M211. But, keep in mind that with vacuum assist brakes you will loose some key features and advantages to having an on board air system. With a hydravac the engine must be running in order to use your power brakes, you will loose the use of your original air horn, air powered windshield wipers, not to mention the ability to pump up your own tires anywhere on the road should the need arise.
If your truck is missing it's compressor, air tanks or air pack then by all means just go with a hydravac. For around town, light use it would do fine. The stock air over hydraulic brake system on these is extremely simple and the manual gives an excellent idea of how everything works and is setup.