Nope, I've had a few 318 68 Barracudas and they had the 5/8 bolts...
My older son's 71 Valiant 318 had 5/8" bolts...
My younger son's 68 Valiant slant had a 1/2" bolt and I had to drill out the hole in the front hanger bracket and get a 5/8" bolt at a local hardware store to put the SS springs on that one...
Look in the 68 service manual in the rear spring torque specs, they even state the v-8 had the larger 5/8" bolts...