Yes it will effect the balance of the rifle.
The effect will of course lighten the muzzle so if it feels a little "muzzle heavy" now, some of that will go away.
As for reducing the pull length, you should realize that the metal will have to be filed down some on the sides and the toe area of the buttplate because the woods shape is tapered toward the wrist of the stock.
This shouldn't be a big problem if there is adaquate material to remove without getting into the "hollowed out area" some of the buttplates have on the inside.
To find out how much material you can remove, take the plate off and check out the inside of it.
Put some masking tape on the side of the stock and sketch a shape the same as the existing cut, but 3/4 inch forward, on the tape.
Measure the distance from the top (heel) of the stock to where the new toe will be. Then use this measurement to see how much of the toe of the plate you will have to cut off.
On some plates, if you remove the necessary amount of metal from the toe, it will break into the hollowed out cavity leaving a gap there when it's installed.
If there is adaquate material, then go for it.
If there is not adaquate material without getting into the hollow, you can consider doing it anyway and filling the gap with something like wood filler.
It may not look too good, but if your happy, that's all that counts.