what box have you got with 4.375 final drive ? i thought series 1 turbo was highest final drive at 4.2 something ?
i had series 1 box in my fiesta track car with 4.7 motorsport final drive which exploded at kames last month and looking for something same as yourself , been told fiesta rs1800 has closest ratios for a standard ford box or ford ka with front mount starter
let me know if you find anything as would be intrested

john