408 tuning with a Wideband sensor

Yep...

I've seen measurements from .393 to .406. I'd go high side at .406.

You may have to flat side the bushing to fit in the opening or open the width of the window it fits in (that's what I do).

You could try tuning the idle before making the bushing, twist in some timing and see how the carb/idle/vacuum reacts. Just don't drive it with your total number being out of bounds.