The Stovebolt Site Forums The Shop Area Engine Shop That was an experience - Rochester B gone wrong

Not sure what you're measuring.
The flange bolt patterns (between centers) are S.A.E. industry standard, not Rochester or Chevrolet.
Small (216, early 235): 2-11/16" (2.6875"), large (late 235, all 261): 2-15/16" (2.9375").

The 216 throttle bore is 1-1/2" (1.500"), 235 is 1-9/16" (1.5625"), 261 is 1-11/16" (1.6875").