340 rear main cap

Dan the man said

I have a simple solution to your problem, just simply replace the rear cap from a 318, as the 318's and the 340's have the same main bearing caps. I'm surprised that no one else thought of this. 318's are everywhere so locating one should be easily done. I can't believe that none of mopar specialists thought of this.

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Actually, 340 caps are not the same as 318. The journal diameter is the same but the 340 caps are a little beefier. However, I think the rear cap is the same.
I would try a thread-sert there or the spray weld option. Not enough meat for a regular heli-coil. You could also braze it up and re-tap it. Braze is some tough stuff.