what gearbox can i use in a cosworth front wheel conversion in a Fiesta ?

 

i believe you can use most gearboxes, i'd use an mtx75 from the rs2k, tough old box, stronger than an ib5 or bc.
you will need a bias box though as the servo will be in the way and you'll need custom driveshafts.
for a straight fit the bc/ib5 will be fine.