it's been a while since i've run a #1666 and i've never done the switch you're talking about, but what is wrong with merely swapping the entire shell? some pictures might help to illustrate your problem or concern.
OK, today I put my Marx 666 shell on my Marx 1666 frame. My problem is removing the smoke box door bracket on the 666 door and installing the 1666 smoke box door bracket. How can the 666 bracket be removed and the 1666 attached? I don't believe there is enough metal to tap and use a screw.
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