Great thread guys...i thought id chime in as, like vvillium3, i am also thinking of making my own intake manifold.
To answer a couple of your questions vvillium3, there have been a couple of centre TB manifolds out there, and ideally the air horn radius should be equal to the redius of the runner. As for the runner taper, length, and plenum size, those are all things i am still wondering about myself.
Phillip aka Blue, has just started an intake design forum on his clubs webpage and there is some great info over there already with lots of pics if anybody is interested. my name is ktzed over there, check it out.
http://www.atlanticz.ca/forum/viewforum.php?f=25
As most of this intake talk is geard towards N/A and i am doing a turbo swap, i was wondering what is required for an ideal turbo intake as compared to N/A. are certain things less/more crucial (runner length/taper, ITB vs single TB, plenum size) when designing a manifold for a turbo engine?