I bought my boat used and then purchased the mast stepper kit. Only one side the center cross bar had been drilled and tapped. The other side had only an indentation where the threaded hole should have been. I just drilled it out and tapped it myself. No problems with it after 3 seasons and I raise and lower the mast about 15-20 times per season. Works wonderfully with the trailer winch.
With that said, if you are nervous about drilling and tapping, and if you have wing seats, you could just attach the line from the mast stepper to the wing seat bracket. Really, you just need an anchor point on the side of the boat to stabilize the mast while in motion up or down from sideways movement.