Re: Iron carb throttle shaft replacement & bushings
A reamer is what I would use. The carb is too odd shaped to be able to hold it properly in the mill. And getting the holes in the carb casting in the proper place is important, but more important is getting both bushings on each side dead nuts square and even with each other. A extra long reamer is best to do that.
If the shaft is a standard size, I would find brass bushings at McMaster Carr that had the proper id for the throttle shaft, and had a reasonable OD. And then get a couple of reamers to ream it out to the OD of the bushings, either .0005 under or ream them to the same size and epoxy the bushings in place.
Then of course the butterfly is going to have to be fiddled with a reset so it fits the throttle bore nicely again.
If the new bushings are a little bit wider than the wear points on the old shaft, they should ride on a new part of the old shaft and might be ok as is.