I'm not exactly sure what is meant by a block off plate, but I'm assuming it just takes the spot where the pull starter block/assembly is.

I believe it would be completely alright to use a starter box even with the pull start is on. When the engine is running there is a one way bearing in the pull start assembly that “disengages” the pull start from the rotating engine. If this was not there the engine would just rip the pull start apart. The starter box would be rotating/starting the engine in the same direction as the engine would be running. (obviously) so the pull start would be “disengaged” from the engine while starting. However, if you hook up the starter box with reversed polarity, you will rip the pull start assembly apart. So just make sure the starter wheel is rotating in the proper direction.

Ryan