I bought a 55 first 1/2 ton with a 261 in it. Rebuilt the carburetor and it ran terrible. I learned the hard way.
It had a badly rusted (inside) fuel tank which means I had rust in the fuel lines, fuel pump, fuel filter and my newly rebuilt carburetor.
Replaced all of it and rebuilt the carburetor. Filled it with Non-Ethanol gas. Much better now but not ideal.
Next step is to replace the Rochester B with a Zenith.
For a Zenith, I would go to The Carburetor Shop website and see what model Zenith is listed for your truck.
There is a round brass tag on the top of your carburetor that has a stamped ID number.
For the ID number there is a matching kit number listed on the website as well as a phone number to call.
Carbking is much better on this than I am but you may need a few special tools.

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."