It depends on where you are putting the weight. If I’m handing it from tubing I use Grade 8 5/16″ minimum. I also go nuts and double nut with blue Lock Tight and drilled for safety wire. You can get drill blocks from any of the big kart shops on line.
If your are mounting to seat I use carriage bolts 5/16″ minimum. Double nutted, Lock Tight, and drilled for safety wire.
This is probably over kill, but I don’t want to get hit by a 5 lb. block of lead. Paint the lead white before you mount it. If it dose come off everyone else has a better chance of seeing it on the track.