1947 Ford Tonner Pickup REAR Fenders
 

Is Black WELT either "Period Correct" or necessary for installation between rear fenders & pickup box?

Re: 1947 Ford Tonner Pickup REAR Fenders
 

Necessary? Not certain about that.
Was it originally installed? Yes. Understanding how automotive companies operate, if it wasn't thought to be necessary, it would not have been installed.
Black? Beginning in 1946, the passenger cars had the fender welt painted to match the body color.
I'm not certain if the trucks had matching welt or not.

If you are simply putting your truck together as a "driver", you may find welting useful. It will fill the "gaps" (fitment) between the box and fenders.
If you are actually restoring your truck, you will need to install welt.