It's a bad idea to carry concrete blocks or wheel weights in a truck. Wait til the first time you ditch it going about 40 and it stops real quick in a snow bank or ditch. It caves in the front of the bed and possibly more. My dads old truck did just that. Caved in the front of the bed, damaged the back of the cab enough to crack the rear window. Plus in a more serious wreck, I wouldn't want those blocks hittin me in the back of the head.
Sand bags or loose sand would be my choice, but I never use anything in my truck for weight.
GB