Your current ball hitch is NOT meant to accept a lunette(pintle) ring with an additional piece, it is designed that way to give it proper support for up to a 25,000lb trailer.
If you want a 40,000lb pintle hitch you will probably have to get a fabrication shop to make a solid mount that bolts on over the channel mount, using all of those existing bolt holes.
Here is a link to a thread on fabricating different mounts, including a link to the CAD files I gave a local shop to water-jet cut the pieces for me to build mine.
See:
http://www.truckconversion.net/forum...er-hitch-7581/
I accept no responsibility for the design or construction of any hitch platform, and you would need to verify the measurements for yourself.