I agree with the others, whatever you do gearbox-wise, it would pay to make sure that when they fitted the 1098 they changed the axle / diff to suit. You need a 4.22 diff for the 1098, then you should get over 70 out of her.I’ve driven another car one and it seems that the gearbox on my is geared to the point that it’s at full revs at 60 50 but shoots up hills
The later axle casing has the filler plug in the back of it, whereas the earlier one does not, on earlier diffs the filler plug was in the side of the diff.
If you still decide to change the gearbox, advertise the 1098 one on here, you'll find plenty of takers.
Best wishes,
Mike.