Question is simple, answer is NOT simple.
First part of the question already answered by Pre'68Dave.
Will Zeniths fit a 235? Depends.
Back in the early years of the automobile, the Society of Automotive Engineers (S.A.E.) proposed standards for carburetor mounting flanges.
Size 1 (nominal 1 inch carburetor) mounting c-c 2 3/8 inches
Size 2 (nominal 1 1/4 inch carburetor) mounting c-c 2 11/16 inches
Size 3 (nominal 1 1/2 inch carburetor) mounting c-c 2 15/16 inches
Size 4 (nominal 1 3/4 inch carburetor) mounting c-c 3 5/16 inches
Size 5 (nominal 2 inch carburetor) mounting c-c 3 9/16 inches
The sizes go up to a size 9 for updraft carbs, but I have not seen any downdrafts this large.
To add to Pre'68Dave's post, the 1950 and newer 235 used a size 3 flange, whereas the 216 used the size 2 flange.
Carter has made carbs in sizes 1, 2, and 3
Holley has made carbs in sizes 1, 2, and 3
Rochester has made carbs in sizes 2 and 3
Stromberg has made carbs in sizes 1, 2, 3, and 4
Zenith has made carbs in sizes 1, 2, 3, 4, and 5
G.M.C. 270's DEPENDING ON THE YEAR AND MODEL used size 2, size 3, AND an updraft carburetor.
So, will a Zenith for a G.M.C. fit a 235? Not if it is a size 5 designed for a 707 CID 6 cylinder.
Will a Zenith for a G.M.C. 270 fit a 235? If it is a size 3 downdraft, yes it will fit, but will probably not work well if the 235 is stock. A Zenith from a G.M.C. 236 would work much better on the 235. A size 2 Zenith or an updraft Zenith from a G.M.C. 270 would not fit.
As a general rule, when migrating an O.E. (original equipment) carburetor to a mostly stock engine, then the donor engine should be within +- (plus or minus) 3 percent of the donee engine FOR BEST RESULTS. 3 percent of 235 is about 7. Therefore, one could migrate an O.E. carburetor from engines of about 228 to engines of about 242 CID to a 235 and expect good results.
For the record, I.H.C. had both a 233 and a 240.
If the 235 is HIGHLY modified, then larger carbs may be used.
The size 3 Zenith from a G.M.C. 270 is an excellent choice for a slightly to moderately modified 261.
Jon.
