I believe the mega shifter has a stop screw that if in, will lock you out of the fourth position, so you can use it on a "3 speed AOD". If you take the screw out, you could inadvertently try to push the selector lever passed first.
Thanks for sharing your set up and link to the kit. I've printed out this kit to file, sounds like a great option. What year is your 518, is it a 46re that you converted? Thanks!