I wouldnt touch the lower as far as crossection. its already big and going to those SHORT SHORT runners with plenums located pretty much right above would be really hard on low end torque. My advice would be to use the stock lower un ported and to keep the diameter consistant from lower to upper.
You have use a non corrosive metal like alumnium magnisium or stainless to make the intake manifold and that requires a tig welder pretty much. Some very high end migs can be used but they are alot of trouble trust me.
If you go with a corrosive metal like a36 or 1010 or 4340 make SURE you ceramic coat or paint with something that isnt going to flake under the heat change or corrode away with the fuel and air or your valves will be getting quite the unhealty snack.
Iv been through the trials of metal working for the last 6 months. This stuff is not nearly as easy in real life as it is in concept.
