This is what I used. It's 1 1/4 " with 5/8 by pass to go to the water pump. With in reason this unit could be installed down stream of the intake manifold outlet and a by pass hose back to the water pump. Pay no mind to the 90 I soldered to the by pass port. I couldn't find a part number, but if you Google inline thermostats images, lots of them, you may see it and find a supplier.
The Contour/SVO Cobra water neck you've shown is the exact one I'm using. I removed the outer cup (on left side of the photo), swapped out the thermostat, reassembled it, and it's been working for several years now. The first photo below (top) is as seen from under the car, explaining the reversed plumbing from top view in the second photo.