Can't say for sure about your Stevens action but a lot of them have different length screws for the front and for the rear of the action. It's easy to check the depth of the threads in the action. Just about all micrometers have a depth rod on them and you can check the depth of the threads in the action. Then insert the screws into the base, hold them fully into the hole and measure how much sticks out. This should tell you how close the screws are to using all of the threads in the action or if they are too short or if you've got the wrong screws in the wrong holes.
Just don't get the screws too long for the depth of the action holes or you will either bind the bolt or not be able to tighten the base down fully.
Also visually check the threads in the action to make sure they are not damaged to the point of no return. You could always have a smith drill and tap them for a larger size if needed.