The "small" (S.A.E. #2) 2 bolt carburetor flange for all 216 and early 235 is 2-11/16" on centers.
The "large" (S.A.E. #3) flange for later 235 and all 261 is 2-15/16".
Easier to measure: IIRC the studs are 5/16", so the outside distance across the flanges is 3" and 3-1/4".