1967 Chevy C10 transmission question?

[Simbone47]

First time posting. Absolutely neat forum!
I am looking at buying a '67 C-10. Of course it is a 3 speed on the column. My question is on unsynchronized transmissions. I am familiar with the double clutch technique but what I am told by the seller is that you CANNOT downshift to first gear until the vehicle is at a full stop. Is this right? My father spent years driving unsynched transmissions and promises me that it SHOULD downshift to first while double clutching or something is wrong with it. Thanks for any and all input.
Chris