My 270 is opposite of your situation. The haed is small port and the intake is big port. The rings transition from large to small and I feel that for the street it is actually a benefit because it probably increases velocity entering the port. I don't know if that would work going the other way. I would not feel bad at all hogging out the intake to do what you want it to do. That is what it was built for, the reason it exists.
I have several Zenith 28/228 carbs. I will use 2 on my Chevy four cylinder. They were used a lot in the "Day". They have separate and changeable venturies and some have external adjustable main jets. I have used then in pairs in the past but have been told by a carb expert that it is hard to tune more than one at a time. I like them.
Yes, there is power to be gained with headers. To work best they should be tuned to the rest of your build and the use the engine will actually see.
