IMO I would keep inside diameter the same and go fairly thin on the tubing especially with gusseting; no sense in adding weight. The tubing gauge from the early 90's Geo Prizm/Toyota Corrolla intake should be fine. You may want to look up some photos of these manifolds as it could be an alternative to the over the top (which is a GREAT design and will yield good low end torque prior to boost).
The engine will be used for low end accelleration/cruise, not just boost and a good normally aspirated design should be adhered to for optimization over the entire range. Log style exhaust manifolds should be avoided as well.