Another thing that I wish I would've done that you may want to consider is building a flat surface across the top of the smoker tank on the outside the entire length of the smoker. If you are wanting to cook for large crowds, sometimes food gets done too early and you need to keep it warm until it's time to serve. The heat radiating from the smoker will keep the food warm while it sits on this flat surface. I've needed this several times and I wish I had it.
As far as the air inlet up high, I have no idea if it is beneficial or not. I have heard of guys doing this and they say it helps move the heat across pretty good, but I have never tried it. If it were me, I would build the smoker without this air inlet and add it if you have trouble getting the CC up to temp. But with this design, I have never had that problem. The less holes you have in the smoker, the less chance for losing heat.
The opening of the wall from the FB to the CC; I would error on the gap being too big rather than too small. With experience from my smoker that is designed like this, I have found that all you really need is for the wall to keep the flame off the food. If the gap is "too big" you will just have to build a smaller fire to get to the temperature you want in the CC, which means using less wood and running more efficiently. I am actually considering cutting some of the wall in my smoker out to allow more heat to move across.
Oh and one more thing, keep your eyes peeled for a jon boat trailer. They are perfect size for this type of smoker build. That's what I used and I was able to build running boards the length of the smoker and turn the back into a storage box. The running boards act as a counter, and so do the doors of the storage box when they are opened. They are usually pretty cheap (I got mine for $140). Considering your tank is much thicker metal than mine, you might have to reinforce the frame of the trailer if you do infact go with a jon boat trailer, but I would think it wouldn't need very much reinforcing at all.