Many many years ago I put a 1.6GSi onto a standard 1.2 gearbox, used to wheelspin in 4th gear & didn't need 1st or 2nd!
However there will be lots of pressure/torque/power going through something that is not quite designed to do so & quite often won't last long
Also consider your engine, although it'll be working easier, it'll also be working faster... It'll get up to the red line in no time & you'll have the tendency to hold it on the line simply due to wanting more power and speed.
You'll have a massive rush of power for a few seconds, then nothing more than a screaming marvel of engineering waiting to go bang
If you get a specialist you could leave your final gears, 5 & 6 maybe, and lower the ratio of 2, 3 & 4 have the best of both worlds