I was building ITB's for my Miata using a cut stock manifold with GSXR600 tb's connected with heavy duty couplings. There are quite a few Miata guys dd'ing with ITB's, you might want to check out some of their pros and cons as it's just general tech not Miata specific. ITB's require more tuning then just a aftermarket IM, and a lot more tinkering with.