Welcome to the Stovebolt community.
Hundreds of un-enlightened people have cast these reliable old 235 engines aside to put boring crate 350s in their truck. A pat on the back to you for not doing so.
That said, it should be easy enough to find someone with an old 235-261 laying around you could raid for parts.
I'm sure you already know the pump itself should be a new one for your rebuilt engine.
Good luck with your project. If you run into any problems, use the search feature of this board. Chances are someone has already solved it. Keep us posted on the project
