Trailing arms thickness and lengths are the same for short and long bed 1/2 and 3/4 ton to my knowledge.
I recently upgraded to 3/4ton springs in my '63. I'm so happy with the result. I didn't notice that much change in the ride (probably because the front springs are so broken in). I use my truck to load up 600-1000lbs of firewood regularly sometimes getting pretty close to the bumpstops (worn out rear springs too, I'm sure). Now my truck doesn't squat nearly as bad as before.
I would recommend ordering a new pair of rear 3/4ton springs through rockauto and installing them. They aren't that expensive, and installation isn't that difficult if you take your time and have some jackstands. If the ride is too rough, you can just put the old 1/2 ton springs back in.
