Installing 5th wheel hitch in K30

[gloriavoxdei]

I've been looking around both here and on the internet and I can't find any information on putting a 5th wheel in the bed of my '85 K30. Yeah, there's plenty of stuff out there for newer trucks but they all have bracket kits etc. I've never seen a bracket kit for a square body so how do I install the rail? Do I just bolt them to the bed or is there an underbed system? Thanks for any help and pics you can provide for me.