I don't think they will mark your card for the mounting. The last stock seat I made had nuts welded to the seat stiffener so the four screws that would hold the slider base could be removed. Once removed, the slider base can be lined up to your liking.
When flipped over the seat looks like this:
My base slider was pre-drilled, and was not drilled correctly. I chose two holes and drilled the others rather than make more of a project out an already complicated project. But, you can see the slider base is not centered. It may not need centering. I would install the tray on the slider base and see where you want it when assembled. Also, consider some type of securing or locking mechanism. Nothing fancy is necessary, but a simple key lock can be installed on either end' I believe, and a slot installed in the slider base. The tang of the lock can be modified to fit what you wish to do.
David