There are quite a few different designs of 'cnc 3040' out there, would it be possible for you to post a few pics of your specific machine and tells which way X and Y is on your machine?

If you are adding supported rail anyway it would be better to get longer rails for the gantry to ride on and double the distance between the bearing blocks by reinforcing the legs of the gantry. That will help a lot with gantry tipping backwards problem.