Don't get tied up on the B. It's one of those things that require time to research, rebuild correctly only to have it fall on it's face. But one can be made to work. I would hang on to the carburetor in case you want to go a different direction in the future are if you need it for the other (or another) 261. You can also give it to me..big grin. The 235 and 261 manifold can interchange.
If not severely warped, it is common for manifolds to be machined flat again. The head on the I6 is not a 12 port. To squeeze out a little more horsepower/torque involves physics "scavenging and pumping effects" with design of manifold size and shape. But, in the end, the result is a small improvement in typical highway driving. Most of us just want the old truck to startup well and run good. A decent MPG is always appreciated. The biggest bang-for-the-buck is to increase cubic inches. In my opinion the best bolt-on option for the 235 engine is a 261.
