http://www.galco.com/techdoc/crou/gn/gnssr_spec.pdfTom,
This is on the link you sent. Click on the specifications link after you click on the first documentation link. Page 1-12 shows that you are correct on your terminal assignments and the derating curves are on the bottom right of the same page. Newark showed at least 3 Wakefield half brick heat sinks of varying fin depths that the SSR could bolt onto. Just remember that something as simple as keeping the fins vertical can improve convective cooling.
Good luck,
Joe
