I have been browsing over on miata.net and have been toying with the idea of a 3.0 swap into a Miata. There wasn't a whole lot of information on the subject and the swap has been done before, but the people who did the swap used an RX7 twin turbo tranny as seen here.
My question is, since the MTX-75 bolts up to a 3.0 and assuming it would fit in the Miata, would it be possible to use it in a RWD configuration and then have a custom made driveshaft fabricated? The info in the archives there was saying something about conversions of FWD to RWD configurations didn't always work.

no, it won't work. The MTX-75 is a completely different layout than you would want. Might I suggest a T-5.

Or, if you must go with something from the MTX-75 family, there was an MT-75 in europe that could be doable as well, but likely expensive to acquire in the states.

Yep. T-5 with an adapter plate.

It'd be easier and cheaper to put in a 302.