Achipmunk, I did say if it isn't going to be stock, do what makes you happy.
The true chassis black has about a 65% gloss factor (satin) I wouldn't put that in the dirt catching flat category. Also over the years some manufacturers changed back and forth on finishes for the under carriage of different vehicles. There was a semi-gloss chassis black that was 80-85% gloss factor (incorrectly called Gloss)that can take it a step up without becoming full gloss.
The reason that manufactures used these finishes is that the steel and castings that were used on the under carriage had no attempt at surface finish quality. All sorts of production and tool marks (and half a century or more later, rust pits).
The rule is: the higher the gloss of the paint, the more the imperfections stand out.
