I don't know of anyone that makes brake kits for a 28 Plymouth spindle. You may have to make your own adapters. another alternative might be a front axle from a 46 to 48 Ford pickup. They had twin leaf springs and there are a lot of brake kits available for them. Another good straight axle to use is the early Ford Econoline. (61 to 66). What's the distance between your springs. I can measure some axles and see what comes closest to yours.
AL