K11 1L 1.4L gearbox 5th gear mod?

[Ste2002]

while out driving my K11 1L, I noticed that the revs where sitting uncomfortably high at 70mph, and I slowed down to 60 to where the engine felt comfortable, and i had a thought about the 1.4L gearboxes, since they are interchangeable with the 1L gearbox and its just the ratio's that are different. would it be possible to strip down my 1L gearbox and fit the 5th gear from the 1.4L gearbox? I'm wondering if anyone has done this before and if its possible.

 

[FoxtrotF]

I don't know if it's the case for the reatyling gearboxes, but pre-restyling cars have a machined primary (or secondary, idk) so you can't change it's gears.