I've been intrigued with the idea of a Toyota 4AGE into a Spitfire and a 5spd trans with it. It's modeled after the Ford Cosworth Racing engines of yesteryear, is lighter than the original motors and powerful (puts out more than my 6cyl gt6) and with modern fuel injection in the light car will get 40+ mpg and not ruin the handling of the car. Maybe you have the same already with the Datsun motor I don't know.
here's a link to something similar
Larry Breitkreitz's 1973 Spitfire, with Toyota 4AGE 4-cylinder
You may have seen this already, Late model midget with that motor.
Toyota Egine Swap