There should be no "block-off" plate for that - the HO didn't have an EGR. Only RENIX did.
If you absolutely need one, you can make one out of any flat piece of plate steel - say 1/8" thick. Use a new gasket when you install, or some RTV Copper.
Have you sorted out what to do about the EGR tube, if you have one? I don't have measurements on those threads anywhere I can think of...