The Jet on a 261 carb is about .010 bigger than the ones on a 235. Also the internal passage between the power ball and the jet is bigger. If you don't end up with a way too rich condition, you will be very lucky. More air yes, more fuel YES.
If its a valued truck and you drive it a lot, get a 235 carb and then sell the 261 carb. They are worth a lot because they are very rare. I just went through this very thing. I had a 261 carb on my 235 test engine and didnt know it. I could not figure out why I had soot everywhere! If you can adjust it where the plugs are gray, you are doing better than me.
