You should really add another plate to the inboard end of the bolt (the bolt will need to be longer). Weld the plate to the harness bar. This will put the bolt in pure shear by eliminating any bending moments. The bolt will also be in double shear. You obviously won't be able to use the plastic cover any longer...

This will be a robust mount. Your acceleration rate in an accident may be slightly higher than a mount with some deflection but the stock belts stretch considerably anyway.

Remember Dale E. died from a compromised belt mount!