As far as I know, not until 1993 or 1994 is that true. Nothing worth
mentioning was interchangeable between the Mazda p.u and ranger p.u.s.
I worked in a junkyard for a while, and according the interchange,
they were completely different vehicles. Most parts on a Mazda p.u
are interchangeable from 85 or 86 to 91 or 92. Some parts are
different through those years, but the trucks are more or less the
same.
As for swapping the mazda engines, I don't know if the mounts
are the same, nor if the transmissions bolt up the same.