Peugeot 205 5th gear swap (8.2% improvement at constant 50 mph)

[Pieter H]

I am considering swapping the 5th gear on our 1996 Toyota Starlet with a 1.3 2E-E engine (C-150 gearbox?) for a 5th gear of a Starlet 1.5 diesel or a Starlet Turbo (C-153 gearbox?) The rpm of the engine is almost 4000 at 120 km/h, but the motor feels strong enough (it's a light car) to have a taller 5th gear. According to Wikipedia the current 5th gear is 0.815 and for the Starlet Turbo gearbox 0.725. Theoretically the rpm would then be 4000/0.815*0.725=3558 rpm, which is stlll quite a lot. Does anybody know whether these 5th gears can be swapped? Or any other options?