Keep in mind the bolt is underneath the cam and near impossible to remove & grind down without removing the cams and timing belt. Also remember they are Torque-To-Yield bolts which means you're not supposed to torque them arbitrarily. Follow the steps in the FSM.

If you have a HG leak, you should replace the headgasket.

The problem is not, and never was, with their bolts. The Mitsu service guy was just BSing you unless they have a new part number they can show you.

ARP makes rod STUDS (not bolts) that will correct this problem.