The 5 speed has the bellhousing made into the trans, unlike the 3 speeds and 4 speeds with a separate bellhousing. So you will need a 5 speed that came from a 300 six, 302, or 351w.
I didn't know they had 3 speeds in 1985 still. What type of hydraulic clutch do you have? Some were external slave, some were internal slave. You may have to swap some stuff there.
You may need some modifications to the driveshaft also. If you find the right truck with the trans you want,, the proper engine and the proper wheelbase, it will be a bolt in. You just need all the right parts. Much easier if you get a donor. Can be cheaper also once you sell off the leftover parts.