mounting a hitch onto a vehicle

huangyhg

mounting a hitch onto a vehicle.
i need some assistance mounting a 5th wheel hitch to a vehicle.
the vehicle is a truck.  sitting across the top of the vehicle frame rails is a 3/4" thick 8" long and 43" wide piece of a36 steel.
for some background:
the frame rails are 43" minus 6" wide so that the steel sits 3" wider on each side.  a piece of 3/8" angle iron is then bolted to the 3/4" plate as well as the frame rails to secure it to the vehicle.  this is the accepted way as per manufacturer's suggestions.  
now i have a 5th wheel hitch with a 1/4" steel base on it.  it is 6" long and 10" wide (pics).  i need to raise this up approximately 6" due to my trailer height when level.
it will be pulling a trailer no heavier than 20,000 lbs. and with a vertical pin weight of no more than 6000 lbs.  we are not talking about an 18 wheeler here.
my thoughts were something like rectangular tubing bolted to the base hitch plate as well as the 3/4" plate  mentioned above with 5/8" grade 8 bolts.  
here is a sample picture:
i should add a few things.  
the steel plate welded to the 5th wheel hitch came with the hitch from the manufacturer.  it is actually 3/8" not 1/4".  they manufacture most of the 5th wheel hitches for 18 wheelers so i trust it.  this one is a mini one rated to 32,000 lbs (i will not be pulling more than 19,000 lbs).  
my rectangles there are 3" by 6" by 1/4".