As far as I can tell (we have a '65 3000, too), the differences extend beyond the carbs. It appears that Ford changed governors in '68 and the linkage to the carb also changed, and I suspect that the bellcrank on the carb was part of the linkage change. Also got to tell you that I've never done a side-by-side with ours and a post-'68, so I could be wrong...what I just wrote about governors and linkage came from hours of reading through the CNH parts catalog. If you do the same, you'll find that the part numbers for governors, linkage, and carbs all changed in '68 (these parts aren't all on the same page, so there's lots of flipping back and forth).
A phone call to the company in the link - Farmers Service Inc. - confirmed that the carbs are not the same. He sells the correct carb for the '65-early '68's.
Now, if I'm right that the only difference in the old & new Zeniths is the bellcrank, then I suppose someone could machine a new one based on an existing part. But if you're going to buy a new Zenith anyway, why not just get the right one to start with?