bmac,
Sorry for not replying earlier, I did, but might not have posted it. Not sure what happened, but I did reply to your PM and will do the same here.
I would return the vents and get flat ones. The ones you have look like they are for brick work, but I'm not positive. The ones I like are made from plastic, or maybe PVC. They hold up real good, have a very clean finish to them and are fairly inexpensive. The octegon shaped ones are easy to trim out and give the building a little something extra when done. Square is ok, but you already have square in there with your windows. Round looks good too, but triming it out takes allot of time.
If you cannot find plastic, aluminum is good too. Avoid wood. The ones I've seen are made of cedar and look ok at first, but then start to come apart. If you have to have wood, take it apart and glue it back together with gorilla glue!!!
I'm not sure where I got the idea that you were leaving your rafter tails exposed. Just thought it up probably. Anyway, covering them up with soffits is an excellent way to clean up the finish of your building. They sell a Luan exterior plywood that's real nice for this. It gives you a satin smooth finish. Of course, just regular 3/8 exterior plywood works too.
Eddie