I have Doug's on my '74 Duster with a 340. The only thing that changed in '73 that could have any effect on header fit compared to earlier years was the change to spool mounts, and really that doesn't change anything.
Why they don't list them for '73 I don't know, but they fit.
If you have a 4 speed, you will need to modify the Z-bar to run the Doug's. But that's true of the TTI long tube headers as well. I think the TTI shorty's are the only headers that don't go below the steering link that also don't require the Z-bar to be altered.