You can pretty much use what you want,as long as it is close to the weight of what it was set up for and depending on your talents as a welder and fabricator.If you go heavier then you need to get into suspension changes,experimentation,etc.Why not the Ford with an automatic?There is loads of hop-up equipment made for that engine,or,if you like,the Ford Ranger 2.3 engine is still the basic Pinto engine.There are aftermarket outfits that sell the electronics to use those engines in different applications.That engine,with the 8 plugs and fuel injection coupled with an automatic overdrive will just fly down the road and get 30 MPG while doing it.Finding and figuring out the electronics for the Miata might be a problem.If that engine relys on the ECM to tell it when to fire you could be in for a real plate of worms to work on.
