Actually, 340 caps are not the same as 318. The journal diameter is the sdame 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.
I did read that the 340 caps are a little taller than the 318's. I'd rather have them wider as this would help make the bottom end stronger. I did read that the rear main cap is the same
