The 518 is longer then the 500, I believe. They are the OD versions of a 904 (500) and a 727 (518), so interchange has pretty much the same differences.

That era Dakota did have a rust problem on the main frame rails between the trans crossmember and about a foot and a 1/2 behind where the frame changes from box to C channel. The trans crossmembers bolt to the inside of the boxed frames, Dodge in their infinite wisdom, put large access holes in the outside of the frame to gain access to the crossmember bolts. Those access holes are in perfect alignment with the turned front wheels, so all the salt & water off the front tires can enter the frame with no provision for it to get back out. The rails rot from the inside out, if the top 3/4 of the sides are still good, a new lower sides and bottom can be welded in. Gene