Lets define this a bit tighter, a 5th wheel plate is a minature of what an uncoupled tractor trailer's plate looks like, they are used in camper trailers mostly. A 'gooseneck trailer plate' is a flat plate with a 2 and 5/16th ball mounted in the middle of it, normallyused for horse and stock trailers.
The 5th wheel plate usually has a pair of V shaped inverted towers, each mounted near the wheel wells and centered over the axel. On that tower pair there is a cross bar with the minature plate. There are usually 3 or 4 bolts placed lengthwise with the towers mounting area with a thick back up plate placed under the bed, pre drilled for the bolts.
The gooseneck plate is normally 1/2 inch or thicker plate that is anchored through the bed to the frame via a couple of cross members that bolt to the frame at their ends and to the gooseneck plate's cornors.
Either application should for safety purposes be attached to the frame via at least 2 pieces of chain under the bed, in the event that the upper fasteners shear.
Both applications require that the piviot point be centered in the bed and vertically centered over the axels. Mark out where the piviot point will be and check the measurements to the nearest obstical. I have seen poorly made trailers not have clearances needed to prevent touching the truck bed. Remember that gooseneck and 5th wheel both are pulled with the tail gate up and latched, so height also needs to be noted for clearances. Also use only 'grade 8' bolts with locking nuts for best service, never allow any slack or shift to exist in that they will eventually destroy something.