The Super Flow carb testing fixture is pretty similar to what Brad used, only it comes with a 4 hole spacer.
The spacer has a small hole in the center of it to locate it on the pin in the plate.
Then you just rotate it around to test all 4 holes.

The hole in the plate is 1.750, but the holes in the supplied spacer are only 1.687, which is fine if you're testing carbs that don't have a 1.750 throttle bore.

I found a spacer with 1.750 bores to use on the big carbs, and drilled the hole to locate it on the pin.

I also made one to use with 4500 series carbs.