Harmonic balancer bolt for stock 318 with 80012 balancer

That thread and pitch is right: 3/4" by 16 TPI.

1. Go to any hardware store, or on-line at McMaster-Carr, and buy a grade 8 bolt of the right thread, pitch and length. That size bolt in grade 8 will provide the necessary clamping force and thread strength. A black oxide finish is best (for the reasons below) but is often hard to find locally.
2. You will also need a washer that is thicker than normal, and preferrably hardened; standard washers will not do. All that necessary clamping force will badly deform a standard flat washer in that size. If you have the original one, use that.
3. Or, get the Summit part and cut off what you don't need.
4. Regardless, make sure you get the torque up to 135 ft lbs. It need that level of clamping force for the damper to do its job.
5. If you get a bolt that is cad or zinc plated that will change the clamping force vs torque, so that is why it really best to get one like the Summit or Mancini part; it has the right thread finish.
6. Yes, the ARP bolt is too much $$ for what it does!