I put a V8 350 HP in the 66 with new 3 leaf springs. After the tornado I took out the complete rear end assembly and put it in the 67' body. I noticed the stock spring bushing I used in the front eyes of the leaf springs had become oval in the ID. Allowing the springs to move in the front. This is not a good thing to keep your car tracking correctly, especially on a track. It was allowing about 3/8" or 10mm movement. It only had about 2500 miles of wear.
The one thing the 67' came with was an new set of poly bushings for the rear. This set came with bigger holes in the front bushing and a metal shaft that went in the holes. The standard assembly bolt goes in the metal bushing. With a bigger hole and more resiliant material, the bushings should hold up better. See the example of Poly Bushings.