hJE, I am in the same boat as you. 1978 CB750K with the Kei Hin carbs (PD42B). The 77-78's apparently use a very different kit than any other years and I am findingthem a rare find.
I did contact CB750supply.com and inquired when they would have kits in stock. Here's the reply:
We will be receiving a shipment November 26th. We can send you an email if you would like for us to when the parts arrive.
Thank you,
CB750 Supply
So I am going to try to reuse the bowl gaskets with a thin layer of sealer to help them along. The o rings can be found at a hardware store and the top gaskets can be reused or cut from some thin paper gasket material. The jets and needle are going to be gently and thoroughly cleaned and reused. If that doesn't work, the aforementioned kits are $15 each.