OK, nice sentiments for your Uncle, but think of what the truck will do for you, not a lot if you keep that engine.
Do this, go to the local JY, see if they have a lower milage 302/5.0L and how much it will cost with a run guarantee; compare that to your 255 build.
I for one think your Uncle would appreciate you making the truck a good overall truck and to make it your own.
351W would be nice but they are usually gas hogs, 302 is a little small too but a good step in between.
Add a set of after market heads to a 5.0L and you have a new motor.
If you were to find a 93 Stang around the JY get the engine/computer wiring harness out of it and a 5.0L is in, wired and running with only a few hookups.
Also, future performance mods will be cheaper with a 5.0L in place, when you feed them money they can put out nice HP figures.
Good Luck on the choices and sorry, I do not know a lot about the 255 so I can’t tell you what will interchange. Think about the overall package before making up your mind.