Pat Beirne's Miata has since been "re-converted" into a bio-diesel range extended EV slash series hybrid: past its all electric range, it can operate at speeds up to 80 km/h / 50 mph on the diesel genset.
You hit the nail on the head re: space for batteries. The answer depends a lot on the system voltage you're aiming for. 90v minimum for a 50 mph car, I'd wager.
Here are a couple more from the album:
Joe Harris' 1996 Mazda Miata
Bill Theeringer's 1990 Mada Miata