I believe you have Bendix brakes and that means parts are available. In my experience the 3800s had plenty of brakes. I finally put a hydrovac on mine when I started pulling a trailer so I wouldn't have to use both feet on the brake pedal. My front ones do overheat loaded with repeated stops from 60MPH on one stretch of road with a bunch of stop lights but then the front drums are getting thin. Otherwise I've had no braking problems in the Western mountains with a camper at 9,000 gross. I think you can get a disc kit for both single and dual wheel models for a little over $500. I'm thinking about it.
You could re-power to a big V8. By 58 they were putting 348s and 322 Buick V8s in them. Then by 62 a 409 was an option. All should be a near bolt in swap. You'd be hard pressed to find a 409 but you could put in any big-block. So you have lots of options for power and brakes.
