where i work we have 30' straight deck goose. it is home made and from fron to back it goes from good to what the ??... the neck is 6" X 10" X 1/4" square tube. then the main frame is (on each side) 2 pieces of 8" X 2" channel welded back to back to make a "I" beam. then the cross members are 2" round pipe. it had some really crappy, way undersized tandem torsion axles with some super single float tires. we just put tandem duals 12K axles under it and that is the best part. they just put 16K on it and the frame bowed. my buddy has a 25' superior and it the neck and main frame is 3" X 12" I beam. with 2" X 3" channel for cross members. it's sturdy. it's got tandem dual 13K axles. not sure if this helped or not but here it is.

 

30ft gooseneck with a 15k payload? I built a 20ft gooseneck 3 years ago, put 2 8,000lb axles under it, frame is 8" channel and neck is 10" channel......jack, coupler and all are also made by bulldog. I wouldnt mess trying to reuse an old frame.