One compromise we've had to make in this swap is changing out the stock Miata alternator for something smaller. In our previous Miata swap we used a 1995 Geo Metro alternator and we've decided to go that same route.
The stock alternator mounting position on the Miata is very low on the engine. Because of the narrow MGB frame rails we needed to move it up several inches. We made a throw-away bracket to test the fit as a proof-of-concept and once we were sure everything would fit we manufactured a solid bracket that locates the lower pivot up and in from its factory location. We were able to maintain the stock belt length and re-use the upper bracket.
We also had to swap the Geo and Mazda pulleys. In order to make the Miata pulley fit on the alternator we had a bushing machined to fit.