Basically what you would be looking for is a booster/dual master cylinder kit designed for drum brakes. These kits are available from various after market suppliers. It is installed under the floor and replaces your stock master cylinder. You will also have to run some new brake lines and connect to a vacuum source. Be aware that this is not a substitute for bad brakes. That era 3/4 ton had 12" brakes and when working correctly were fairly good. The big weakness is the master cylinder so an upgrade would be a big improvement. The conversions I have done are by no means "touchy" and work fine in all conditions.
Fred
