First, go to your local parts store and buy a striping kit that puts adhesive tape stripes on your vehicle. Buy one that has two tapes about 3/8 " apart.. Clean the cab around the area you want the stripe. Install the tape on the cab on area to be striped.Peel off the outer layer of the tape, leaving the two stripes on the cab. (it does not matter what colour these are)Take the ends of tape and taper them to show the pointed ends.
Paint between these tapes the colour you want, as soon as the paint is dry, remove the two tapes and you are finished. This process worked for me in painting the Gigilo Green on my '39 Suburban. The stripes end at the point just at the end of the upper wide grille bar. Hope this helps. Stan