Well, your last post changed the answer. You don't really want to shorten the throw, you want to move the position of the lever up.
I think you can adjust the shift rod length, both arms an equal amount, and move the neutral position of the H-box up. That should put every gear postion higher. You may or may not like the result.
If not, then you can modify the transmission levers to shorten the throw also. Try drilling another hole as close to the existing one as possible and moving the rod to the new hole. Nothing to lose as you can always move it back if it isn't to your liking.
The old time floor shift conversions almost always included shorter trans arms to make them 'short throw'.
