I have not seen a site that breaks down all the Zenith carbs like that but I did look up the two numbers in my Zenith data book.
11963 specs are as follows: Model:228BV11, Venturi #33, main jet #31, Power jet #20, well vent #16

11965 specs are as follows: Model:228BV12, Venturi #34, main jet #33, Power jet #18, well vent #15

Both were used on late 50's GMC 270 engines. Jetting and venturi sizing may be too big for your 261. Check mount hole spacing as these were different among the GMC Zenith carbs based on application.

"The Operation of GMC trucks is normally smooth and steady, with no loud rattling, knocking or unusual noises. A good driver will quickly become accustomed to driving a GMC truck and will get the feel of his vehicle regardless of his engineering knowledge."-TM10-1563