Plugging smog holes in heads....

Dartnut said

As cheap and dirty as it sounds, i just used a mig welder to seal mine shut with a quick plug weld.

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If the heads are off, would it be best to put the weld on the inside of the head, which would decrease the air volume slightly? Or might the heat soften the infused hardening of the head (e.g. w/ a ...302 head)?