i have a ford 1.8 with a type nine box and a cortina gt axle, the car struggles on power in 4th and 5th but is fine in the lower gears,

what ratio is the cortina gt axle?

what will be the best diff to get good accelaration and decent top end speed?

oh yeah i have a 1.6 capri diff i could put in do you think this woul be better?

when you say it struggles in 4 and 5, do you mean running out of revs or just not enough pull? If there is a difference between the diffs then i would expect the 1.6 diff to have a shorter ratio (less top end) and so it should help the pull as revs will be higher.