16ga or 14ga works. Any color works. Colors don't matter on simple old truck wiring. Especially since you will be doing a new harness at some point. Colors are to be able to follow and trace (and to some, to be purely correct). Tracing is easy to do with our vintage trucks most of the time. Since you are adding the wires, you know where they will go. Don't sweat the small stuff. Adding a terminal block is a bad idea. It introduces a place for corrosion, can come loose, adds resistance and is unnecessary. Splice with crimp or solder and cover with shrink sleeve............best to run a whole new wire from switch to light if you can. That way at least you are using a continuous color.
Colors used on aftermarket turn signal switches don't always match colors used on wiring harness kits...but that's OK also.


Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn