Hi Carl.
I know that on some vintage GMC and Chevy tailgates, the embossed letters weren't painted. So that's always an option.
But many more were painted.
A technique I would try is dry brushing. I would use a short-bristle, medium-stiffness brush and Tamiya acrylic paint. Lightly load the brush with paint and brush most of it off on a paper towel or such then brush a very light and thin coat on the embossed letters one letter at a time.
Brushing might not be the best term for applying the paint. It's more like highlighting or tinting. It does take some practice.
This will take numerous applications and much patience but the chances of slopping it up are greatly reduced.
I would use Tamiya acrylic paint as an accent for the embossed script over a basecoat of enamel or lacquer. This way any errors can easily be removed with Tamiya acrylic thinner and a firm cotton bud. There will also be almost no chance of damaging the base color if it is lacquer or enamel and it's fully cured.
Tamiya has a selection of cotton buds in a variety of shapes that are perfect for just such an occasion. I have couple of favorite shapes that I always keep on hand.
I hope that helps.
David G.