fifth wheel rails

You didn't specify what hitch.

I currently have a B&W companion that requires a goose-neck mount. If I change trucks, it most likely will require a different under bed mount.

For something like an RBW little rocker and many other brand hitches which use 2 inbed mounting rails, the only thing that would need to be changed would be the frame L brackets.

I have welded the L bracket to the frame and then when changing trucks have used a grinder to cut the welds so I could reuse the bracket on the new truck.

As for your question, not all hitches will fit all mounting rails.

You said you found the kit for the truck. If the kit for the truck was for the hitch you have it should work.