Someone already started down that road, but I'll continue down it:

"Why would anyone put a fuel cell in a Miata ?"

Miata has probably the best located (for safety), best designed tanks of any production vehicle that is raced in IT.

Any cell will be higher (bad), further rearward (bad bad) and unibody strength will be compromised by installation (bad bad bad...plasma cutters & sawzalls don't yield benefits in structural rigidity).

Really.

But that's just my opinion.