Can anyone tell me the correct gloss for the black paint on a 1968 chevelle. I mean the black paint on the tail panel, filler panel, etc. Should it be flat, semi flat, satin, semi gloss, etc.

Satin black seem to be the consensus

I used SEM 39103 Bumper Coater on anything that had paint on it and 39143 Trim Black on anything that was bare metal, they are both the same color and finish. Expensive but worth the cost. Nice thing is when the rocks chip it off just spray it back on.

You should ask one of the Bills. If you have a SS, the lower sheet metal is black. I had the filler panels painted the same gloss as the sheet metal. Not sure if correct gloss but I'm pretty sure it is not flat.

Did you look in the body shop forum??
http://www.chevelles.com/forums/showthread.php?t=171177
http://www.chevelles.com/forums/showthread.php?t=181609
There may be some tips in these for you.
BillL