The only addition I could add to your testing would be to fill an empty squeeze bottle (mustard? ketchup?) with gasoline, and squirt this through the passageway to the pump discharge nozzle to make certain this passageway is open.
Original 261 carbs are neither easy to find or inexpensive when you find them. The commercial rebuilders group them with the smaller 235 carburetor, which will reduce power.
(Opinion) If it were my truck, and I wanted to drive it rather than look at it, the Rochester single barrel would leave the engine as soon as I could get it off, and the truck would sit unstarted (yes, I would look at it for awhile) until I could find a real carburetor. A good choice would be a Zenith from a G.M.C. 270. Stromberg also made some excellent replacements, but they are scarce and expensive.
And Bart, I share your opinion of the Q-Jet and the 2G; some of the finest carburetors ever built anywhere in the world at any time or any price. As to the B, a useful upgrade is to hire a 15-year old to stand on the running board and pour gas into the engine out of a leaky boot!
Jon.
