Forgot to answer the question about placement of the shear webs...

I remember reading an article in RCM some 10yrs ago about a study they did on the relative strength of the wing with various shear web placements:

1. "I" beam
2. Outside of D tube
3. Inside of D tube

I don't remember exactly, but either 2 or 3 came out marginally stronger than the rest according to their test results.

In any case, I agree with their conclusion that in actuall practice, they should all work very well. So ease of construction can weigh in heavier in one's decision to use one type or another. Personally, when I have the time to do it, I use very lightweight vertical grain balsa to fill in the area between the top and bottom spars completely - no gaps between ribs and maximum glue surface for the shear webs. Ounce for ounce of shear web weight added, this is the strongest method.

BTW, if your top and bottom spars are not parallel to each other and you want to use side shear webs, be sure to sand/shape the spars so that the spar surfaces to which the shear webs will attach become parrallel. OTherwise, there will not be adequate glue surface.