I have added a twin slide seat unit to my NED my own design . Can't seem to send a picture but if your interested email me [email protected]  

No experience, but it seems to me that you might be able to make it work.  If it can work on a Northeaster Dory, the Southwester Dory seems to have similar freeboard amidships, if the two comparison drawings on the Southwester Dory page are to the same scale.  One thing to look at is how the high centerboard trunk in the Southwester Dory might be in the way.  You'd have to see if there was sufficient length aft of that to accommodate whatever you plan to use.  The motorwell might be in the way, too, if you have that.

Oh, duh...here's from the Southwester Dory page FAQ:

Can I fit a sliding seat for rowing?

Yes, a sliding seat could slot in aft of the centerboard trunk.  The standard drop-in unit would be very much in the way of operating the boat under sail, so it'd be something used only on special occasions.

Just some thought from somebody who's never used a sliding seat or seen a Southwester Dory....

.....Michael

Thanks Michael. I did see that in the FAQ, but the SW Dory is not listed in the pull-down menu so I thought I would check. I'll pursue the idea...