This is a great resource. I had the same question, since my 74 750 k also says 750 f on the air box, and here is the answer.
By the way, I just switched to #35 from #40 idle jets because the bike always smelled like it was idling rich and it seems to be better.
The idle screws just don't do very much on my carbs though. In fact, the further in they are, the better it runs. This was the same with both pilot jets sizes.