Funny you should post this. I was literally looking into the same thing yesterday and today and searching the site to see if there were any answers. I have '70 Swinger seats in my '68 V100 and was going to order the matching door panels. However, I noticed the rear window cranks are in a different spot on the post cars. The hardtop panels won't work, but it appears that Duster/Demon parts might.
Legendary says the '69 and '70 Valiant and Swinger panels are the same, but they don't look like what's in my '69 Signet, and the window cranks are very differently located between the two models.