As you can see, it leans toward the small size. Personally, I'd stick with the 1 5/8th OD. Internal pumping loses will take their toll on a 1 1/2 X 55 tube. Panics right, the calculators do not take into account the siamese port but with this short of a duration his analogy of trains on a track is applicable. Sometimes you have to work with what you have and make the best of it. You are doing exactly what you need to do, ask questions, make changes, note the results, repeat as necessary!
Maybe Panic can help with the "What do we do with a siamese port" question. The racing community has been waiting for 70+ years for a good answer and he is absolutely tenacious about ferreting them out.
