Some pylons aren't rated for pulling tubes - or at least only small ones with small riders. I have no idea about the Grady setup. But the last thing you want is to be on the fringe of it's rating and have that thing break off and hurdle back towards your riders. My suggestion... look up Turbo Swing.
I second the fact that pylons are not meant for tubes. A simple bridle attached to the aft tow rings is what is recommended.
A bridle will work for towables or skis. Here is an example