Also, as you mention the '55s, keep in mind that there are 2 different trucks for '55. The First Series is essentially a carryover '54, with some improvements (open driveline), and since they were a short production body, can be hard to get some parts for. The '55 Second Series has the same body through '59, with some changes.
On that note, the Second Series '55s did have an option for power brakes, using the same Hydravac system as the larger trucks (optional 3000/4000, std on 5000/6000), though it is somewhat rare. I've only come across maybe 3 pickups with it.
