Although your Cardinal is 'out of warranty' contact them and inquire
"Would the use of a gooseneck void my frame warranty"??
Answer YES
then question is why?
WHY.because pin box structure is NOT designed for the added leverage/stress a gooseneck can apply.
Look at the structural design differences between a gooseneck trailer and a 5th wheel pin box
One uses large beams/heavy supports/bracing-----other uses a wide spread frame with thin/light weight cross pieces and some web bracing.
One designed to carry/handle heavy/high stress loads----other designed to carry/handle much lighter loads spread out across the structure
Gooseneck...for gooseneck trailers
5th wheel hitch...for 5th wheel trailers
Right tool for the right job.