Never seen a Miata with a V8 transplant before.BS till I see it 
http://www.monstermiata.com/[edit]oops, that was the Canuckistan link. Here we go.
Dad's on his second Miata, and two other friends of mine also drive them, although one did admittedly give it to his daughter (he's an OTR driver, no need for a car atm). It must suck to be short and have a small noodle- all kinds of fun cars you aren't secure enough to drive.
Another interesting swap, although I think it was a one-off, was done by Racing Beat a number of years back- IIRC, they swapped a 13B TT into an early Miata. I'm a recovering rotorhead, so an American V8 swap is almost heresy, but I have to admit, either would be a great deal of fun.