I doesn't really matter which wires you cut but if you are going to Earth the Negative terminal on your battery to the chassis, then extend the red one to your relocated switch and Earth the other terminal on the switch to the chassis. At the engine end, the cut-off black wire should be Earthed to the chassis.
This is useful if you are going to fit other elctrical Items.
I not then cut both and extend both to the relocated switch.
X!ZZZZ^+POP!
:>)