If you have a Turnover ball why not get the SuperLite 2600 Single Point? It captures around the gooseneck ball and you don't need to buy an additional ball to rail adapter.
I have the 2600 and like it.
edit add...I'm a bit confused, are you using your existing truck or buying a new truck? If buying new get the factory prep package with pucks and just drop a gooseneck ball in the puck and get the Superlight 2600.
If you buy a GM truck you need the PullRite support kit. GM under bed support rails are not under the gooseneck ball and require additional support. The kit looks like an easy install.
PullRite customer service is pretty good too.