Ive got a fiesta rs1800 with a MTX75 gearbox with escort rs1600 internals in it, it has just been rebuilt but 5th gear seems flat, it accelerates well but when you get to 5th the speed doesnt realy climb

i want to change my box but dont want to mess around with the linkages so the focus st170 and focus rs box are out the question.

what would you recommend?