In addition, if you only swapped the rear brakes to 3/4 ton, which would also require swapping the rear axle, you would end up with a brake system with poor porformance, as the front and rear balance will be upset.
As far as springs go, 3/4 ton springs were a factory option on the '55-'59 trucks. My '55 3200 has 3/4 ton springs on the rear.
