Sorry, I don't have a good way to get pics onto this site. We're both dealing with mental images of the carbs we've worked on. The ones I have done have a shaft that goes all the way through and comes out both sides. I assume, perhaps incorrectly, that yours is the same, but maybe one of the holes is blind. In any case, yes, two bushings with slightly undersize holes that are line-reamed to the diameter of the shaft after installation. This ensures that the bores are in line and that the shaft can fit tightly w/o binding. I would stay away from sanding the shaft. You'll end up with various diameters and, probably, not round even if you use a block, you'll probably end up with taper one or both directions. That is, fat on one end or the other or in the middle. The shafts I dealt with were, miraculously, not worn to any meaningfull degree so that was not a problem. If the shaft is worn out and the holes in the body are egg shaped you have twice the fun and, as much fun as it is, you may want to find an unworn carb. Making a new shaft is doable and, if made a little oversize, you might be able to avoid bushing the carb body altogether. Since, I surmise, this is not a race car engine the slight additional restriction to airflow probably doesn't matter.