I finally had my 68 L34 re-painted about three years ago (July build Butternut yellow with black top). Based on this thread and other resources I had the painter spray the tail panel in 70-80% gloss. This was done with a B/C C/C system so most of the flattener was in the clear. I haven't washed and polished it more than ten or twelve times but I think the tail panel has become more glossy. I avoided putting any polish or swirl remover on the black. Frankly just polishing it with a cloth seems to have made it somewhat more glossy. It still has a slight semi gloss look but against the butternut it really stands out. But according to the above posts my car may have had a glossy tail panel sheen since it was built in hot summer. Folks may want to consider starting with a little more satin look to start, with the idea that as it gets cleaned and polished with time it still retains the semi-gloss look your after.
Allen