Going on an A55 bottom end, the port will be almost flat because that cylinder angles up, but you'll need to figure out a way to get that pipe around the case.
That header isn't too big. FWIW, the Simonini has a 30mm header too. Run the 24mm Mikuni. You won't affect the port timing, as long as you don't alter the port window in the cylinder. If you just match the outside of the port to the header, you'll be fine.