JDM is going to give you a lot of headaches whenever something breaks. I always say keep moving forward when something breaks. If the motor blows, put something better in. Why not just do a v8 swap and ditch the 6 cylinder? Insurance will still think its a v6 so you wont get as shitty of a rate.

Definately take it to another tuner if you've blown 2 motors due to running too lean. You might want to look into fueling issues too, better fuel rails or fuel pressure regulators in case each cylinder is not getting the exact same amount of fuel at WOT.