Relying on distant memory here but the C6 on my 76 Super Camper Special has 1st, 2nd and reverse but no 3rd gear. Some one told me this was odd because 3rd and Rev are. Suposed to share the same valving.

 

3rd and reverse do use the same set of clutches to work so that should rule out a failed direct clutch. If the transmission will shift from 1st to 2nd when driving in the D range this could indicate a stuck 2-3 shift valve in the valve body.

If you can only achieve a 1-2 shift by placing the shifter in 2nd then this would indicate that the transmission is just not shifting, this could be because of no vacuum to the modulator or a stuck governor valve.