Hi John,
A couple of things. If the PPG Omni is catalyzed, then you can paint anything over it after about 3 days. Sand it lightly. If the PPG Omni is non-catalyzed (not hardened, if you prefer), then I'd wait a week or two before painting anything over it. Non-catalyzed acrylic enamel has a tendency to lift if you paint over it before it has completely dried. And it dries more slowly than you might think. I believe that is the reason PPG gives no re-coat data for non-catalyzed Omni. As for the question about semi-gloss or gloss, I think the original was gloss but I've seen plenty of reproduction hubcaps and grilles done in semi-gloss. I don't think you can make a mistake. Semi-gloss is a little more forgiving, for what it may be worth.
I've seen people apply the black vinyl letters and leave them alone. I think if I wanted to paint, I'd prime first, sand down to 500 or 600 wet, spray black where you wanted the letters, let that dry very well, apply the letters as a mask and then spray a non-sanding sealer over the black (and mask naturally) to keep it from showing in the white. PPG makes several of these. You won't need but 1 or maybe 2 coats of this and you should not need to thin it. Once that's done, then I'd spray the Omni white and be done. If you have any questions, you might ask the place where you bought the paint. If you use primer as your sealer, mix twice the normal amount of thinner, spray it wet and try to shoot it so you don't need to sand at all. You obviously don't want to sand with your mask in place...
