If you have a 2008 3500, it must be a long bed dually? Suggest you opt for a conventional fifth wheel hitch of some known brand.
Gooseneck conversions put added stress on the frames point where the upper frame is connected to the main frame--if the angle is not gusseted properly, potential for weld/frame failure at that point. I tried this on my Suites and literally broke the kingpin weld off the hitch box (luckily I was in my yard at the time).
Not too bad an idea if the fiver is a light one.
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired