That is a bummer, seems like it is always a gamble with an old carb. I understand that you don't see the right ones too often and when you see something on ebay pop up, my instinct is to buy it because of the long hunt versus need on the project. It happens to all of us trying to find parts for these trucks. Wonder if carb king would have the base you need.

1953 Chevrolet 3100
261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done
In the DITY Gallery
Video of the 261 running

1964 GMC 1000
305 Big Block V6, sm420, the next cab off restoration