I've been reading this site:

http://www.higgins340.fsnet.co.uk/mods.html

and at someway it says that with a 480 series gear lever, the changes are now 90% faster...

Why that difference?

I have two 400 series gear levels lying arround (one form a 440 Turbo and another from a GL)

I realy dont thing a short lever makes that much difference. My old man can do an extreamly quick change with the old amazon-the gear lever is over 2foot long (600 mm) !!!!!!

Yes, and the gears in the 340 are not that bad... but for put something better we are always interested

If the top half is short it will change quicker but also if the section below the pivot is longer it will decrease the length of the throw!! i looked into this briefly but never got it totally finished!!! As i will get the chance soon i will do though!!
Looking at the two sticks 400 and 300 togetheer you need to get the metal pivot/ball from the 300 one to replace the 400 one which is plastic and the wrong size. Dependent on where the ball is placed down the gearstick will depend on the overall change it will hyave to gear changes.
Cheers Adam

Is it hard to do?

If you have the tools no, basically you will have to cut the 300 lever top and bottom of the metal ball/pivot redrill it and weld it to the 400 lever then mod the reverse pull up to fit!!!
Cheers Adam

Hum...

Welding... Never tried to do it.. Maybe someday