Its a squeeze to even fit the 2 litre lump under that bonnet let alone a 6 speed box (i fitted new bulbs to a 172 the other day and there was space for my little finger in that engine bay and prescious little else!), they only just managed to fit both in the 306 gti-6 and thats a bigger car. Still though with the 306 the turning circle is as good as an oil tankers! 6 speeds would be good, especially in keeping the car right in its powerband, but IMO it would be too impractical to do.