340 Duster the difference between a Superior and a Capitol are a series of refinements. The '26 & '27 have a lot in common. Engine, body, wheel base etc. The '27 AA was the start of a new kind of ID for Chevy. 27=AA, 28=AB, 29=AC, 30=AD 31=AE and so on.
Gary Wallace has the needle & seat.
http://www.20schevyparts.com/You may also want to check out the filling station.
www.FILLINGSTATION.COMThere are others on this site who will be able to answer many of your questions.
Good luck and I hope this helps.