speedo cable doesn't fit

'73 Duster with 727. Bought a new speedo cable (has plastic clip on one end to back of speedometer) but the other end doesn't fit the transmission. The old one I unscrewed used a 1" nut which was a bit larger than this new one (which has a knurled end to screw onto the tranny instead of a hex nut) I've looked all over the internet and only come up with the same type of cable I bought.

Here's a pic that might help clear things up.
http://imgur.com/dnMHUTY