Nick,
I can't answer a lot of your questions because my 51 B3B dual carb set is not yet fully on the road although it does run and drive. However, I can tell you that I've also done a lot of reading and research as you have and most of what I've found is dual carbs will help with mileage and performance, assuming you've upgraded the exhaust.
Based on what I've read and what I've experienced so far just driving my truck up and down my driveway and around my property I don't think you can't go wrong with the dual carb conversion. I would highly recommend converting to 3.73 which is what I have. My 218 is bored .060" over making it a 226 with a 230 cam. The head might have been shaved slightly but only enough to ensure it was flat.
My setup is dual carbs on a Rusty Hope modified intake with split 2" exhaust into to the muffler with a single 2" outlet from the muffler, i.e. 2 in 1 out. It was surprisingly easy to get the idle down to around 500 rpm.
I do think your biggest improvement would come from a change to 3.73. The best way to improve mileage is to reduce rpms.
Brad